My son has one of the 110cc Chinese atvs that alot of people bought for their kids this past chritmas. It will run fine for 20-25 minutes after it has set up for a couple of hours. It has gas and spark. The only way to get it to start after it has died is to wait. Anyone have any suggestions??  

do a valve adjustment, 80% chance you have tight valves  

when's the last time you adjusted the valves? never? start with a valve adjustment , set both valves at .004  

Try new spark plugs.Good ones and not the same ones from were you got the bike.The cut out switch can play up as well. The air filter needs cleaning all the time. Look for shorts in the wiring.I had trouble with the Clutch for 49cc atv quad bike engine,it needed replacing after only 2 hours of running.I think the idle was too high and it was sticking when it got too hot,after it cooled down the bike was good to start and run again. That sounds what you are having.The second one I got about 4 tank fulls out of it and now it's doing the same thing.There are different brands you could try and see if you can get a better part it might last longer.  

I had a clutch rebuilt with truck brake pads cut and glued to fit when the old one were removed and it has never worn out since.The ones they come with were out fast.You could try taking the clutch to a brake rebuilder like I did.