1.4A shouldn't trip the protection circuit on a cell. I think 1.4A should be underpowering the emitter though since the P7 should be able to handle more. Are you certain the light was on the "High" mode when you took that measurement?
<you already answered the question before I got to ask! 
>
Sorry, so mnay people asking me to do things.
I just sat for over a minute with the DMM and the light wouldnt cut out. If i put the tailcap back on and try it it does.
So it must be a problem with the tailcap?
1.4A is pretty low. No protection circuit should be tripping at that current draw. I'd have expected something more like 3A.
Make sure the black lead of the multimeter is in the Com socket and the red one in the 10A or whatever current socket. Set the meter to the 10A/whatever range.
Apply the black lead to the base of the cells, the red one to some unanodised part of the body. I'd be expecting above 2A, possibly above 3.
Looks like it is. Strange behaviour for a tailcap, though.
Did you already take a look inside?
did this, I tried it on my P8 XM-L T6 as a comparison. The P8 draws 1.02 according to my DMM.
I have tried all the settings on it and they all say the same number but with the decimal place lower or higher.
Yep but I dont know what I should be looking for. There is no loose bits or anything that looks like it would cause problems.
I was just wondering because it has happened to me that I disassembled a misbehaving tailcap, found nothing suspicious and reassembled it in a rather frustrated mood just to find out that it worked flawlessly afterwards. (same principle as a sham arthroscopy)
Just took it apart again and what i noticed was the aluminium ring that has the 2 holes that unscrews was loose. Now i have turned it on and off maybe 50 times since i took it apart and tightened it up really tight, seems a little wierd.
Anyway even after it being tightened its just the same.
Try and measure the resistance in the tailcap switch.
Set your multimeter on the omega symbol and also move the red lead to the right socket.
Here you go:
http://www.manafont.com/product_info.php/replacement-tailcap-switch-for-ultrafire-502b-p-5850
Replacement 502b tailcap/switch replacements from MF for $2.86US. Cheaper than shipping your light back to DX... Buy two, one for the second light you referenced, along with a new drop-in of your choice so get a second type of light out of the deal. I'll recommend the 3-mode XML Ultrafire drop-in if you want brightness or an XR-E R2 if you want a light that will throw farther at lower power levels. Or a Blue, Red, Green, or UV drop-in. Or an XP-G for a good mix of flood and throw, or one with a secondary Red emitter to help you preserve your night vision.There's even a 3W infra-red module to use if you have any night vision equipment.
http://www.manafont.com/advanced_search_result.php?keywords=module&log=&x=6&y=7
Sometimes switches get flaky when overtightened. Back it off a little and see if that helps.
Does it cut out if you connect the battery negative to the body with a paperclip or the like? Is it brighter or dimmer than with the switch?
If it doesn't, the problem is somehow with the switch - I'd suspect a bad solder joint somewhere. If this is the case since you're in the UK I can swap you a known good switch for your (possibly dodgy) one as it'd be easy enough for me to fix it. PM me and we'll hopefully get you sorted.
Hm, I'm slowly running out of ideas (not that I had too many to begin with).
I assume you have tried to shake/tap the light after it cut out with the switch still in the "on" position? Does it flicker when you do this? Or does it stay dark no matter how hard you shake it?
EDIT: Looks like Don already came to the rescue. Did I mention that I'm a slow typer?
So if i speculate that your P8 draws 2,8A-3A it will probably be true for your 501B with P7 that go over 3,6A+ and thus tripping the pcb. Might be the dmm (digital multi meter) or the thin leads that show you a lower reading than it actually is (we've seen many such problems).
Whats a P8? A c8 clone named Palight?
Oops i meant C8. Which by the way is such a lovely and well made torch, got it from manafont and its so much higher quality than the 501.
A 502b tailcap does not fit onto a 501b flashlight they are quite different.
If your DMM does not cope well with PWM (pulse width modulation) it will give you incorrect current readings. I have a very expensive Fluke that I cannot measure flashlight currents with.
Go with Don's suggestion as he is over here and very able to resolve your problems.
I have 501b's and C8's and yes the C8 is better but it costs more and gives a different type of beam i.e. C8 = bigger head = more of a thrower than a flooder and reverse is true of the 501b.
Switch the driver to either:
http://kaidomain.com/product/details.S020064
http://kaidomain.com/product/details.S020073
or
http://kaidomain.com/product/details.S010105 <- being the cheapest by piece. 5 usual modes current is roughly 2A at max which would make sense in such host anyway.
You/re probably out of luck using the stock driver as even if you get an IMR cell that could supply the asked current is probably severely overdriven and you're very close to see the early emitter demise. It looks rather blue to me either the camera does it's tricks or it is a poor and very cold color temperature emitter and (i suspect) heavily overdriven and screams in blue. :)
Problem solved.
Need help replacing a driver? Look in the DIY section on BLF, there are some tutorials already and it's really easy to do.
That applies if you rule out the possible switch problem (you can short the battery to the body with a paperclip, screwdriver, nailclipper whatnot with the tailcap being off to check if the switch is annoying you. I doubt it but you can never know.)
Oops...
Here's the 501b tailcap, sorry about that. For some reason, I thought we were discussing the 502b. I guess from a different thread...
http://www.manafont.com/product_info.php/replacement-tailcap-switch-for-ultrafire-501b-p-5849