I've just getting a 1200 one and used it the neet it was great but after 3 hours started to dull then brighten in spasms , sussed it tho , has yours got a rubber battery back on it like mine ? If so the battery's move slightly while on your head and it must be running on one battery , which it will as I've tried in the house , I left it on for a hour + to see how long it would last after fishing and it was dulling until I picked it up and by accident held the rubber cover which rehoused the battery's propery and it lit up bright again .
Try pushing the battery's firmly in when it dulls , just squeeze the rubber back to resite them , if it's not that then I'm buggered if I know but there're cheap as chips and cheaply made so could be a wiring problem ?????