well, I BOUGHT a new aftermarket CDI(actually dealer installed) and it failed in a few hours of use?
and many others have had issues with aftermarket electronic's more than NOT, just like JUNK cheap O china carbs, some get lucky, many DON"T!
SO> take info that as you wish!
new doesn't always mean good!
if you have GOOD fuel, fuel flow, GOOD carb, good spark,, and its dying out on you when it gets HOT, does the oil temp sensor work?
have you tested it, maybe the on dash light bulb is burnt out, and just not indicating its throwing the oil temp switch which is shutting things down
if its NOT the oil temp switch, and again you have all the above, compression issue's could be your problem, these are NOT new atv;'s so, odds are they seen a lot of use and wear and tear!
I am already guessing its NOT your fuel cap venting issue(which can happen when a motor gets hot and the vent isn;t working correctly) since you said you used a different source tank to run and still had same issue
but if not, I would check cap and make sure its venting properly!