Well, first off this certainly could be my crap soldering. But the connections may look like crap but they seem to work.
That said, running a XP-L HI off a single mode 2 amp driver from Mtn Electronics (its what I have). Output is dim but if the ground connection is weakened output increases dramatically (seems to get the full 2 amps). Shaking the light has little effect, same with smacks or drops so no lose connections. Loosening the tail cap or head to JUST the right point causes the led to go to full output. Upon power on output is full power for a tenth of a second or so.
How is the driver held in? Retaining ring, soldered, press fit? Have you tried a shorter battery to put less pressure on the driver when the body is fully screwed in? As MsDNF said it sounds as if the driver isn’t making good contact when the body is tightened down.
Body is a Convoy M1 with integrated shelf. Driver is held into the body by a screw in retaining ring. I can achieve the same situation by removing the driver from the head (long wires) and directly holding it against the tube and battery. Battery is a Samsung 30Q and although certainly long it does not seem to be too long.
Mentioned before, the led always goes to full brightness for a hundredth of a second upon startup, then output drops. This somewhat makes me think the driver is oversaturated (1amp driver boosted to 2amp by Mtn elec).
Or possibly the soldering job on the leads coming from the driver are not too good, causing contact when pressure is relieved and losing contact when pressure is applied. Or as the others have said could be your retaining ring is not clamping the driver down enough. So actually loosening the body INCREASES the ground because the ground ring on the driver is now making full contact with the retaining ring.
Look around the retaining ring and see if any component or connection is close to it.
Simply filing a little material off around the ring would fix the issue if that’s what the problem is.
