when it’s switched on, off, and on again, or I change modes, it seems to flicker like strobe for some few seconds, stays on the same mode, then the light kinda fades away like the Battery is low.
When I change batteries it’s ok , but once off and on again it does it again.
I have fought with this a time or 2, some lights seem worse than others. Mine have always been either the driver or switch retaining ring are not tight enough to make a great connection. In any case it is usually a weak BAT- connection for me.
I bolded part of your post for relevance. The problem “could be” any part of your light as far as we know. As other members have mentioned - please post detailed information about your custom flashlight.
Source of flashlight
Driver
LED
Battery (you’ve already posted this)
mods (bypassed springs, sanding, new switch, adding triples, etc)