My dirt bike wont start!

ive taken it to three mechanics one of them is currently trying to figure out the problem. It starts first kick and then as soon as it warms up or i ride it it wont start.Now sometimes it wont start at all. its a rmz 250 year is 2004 he has already tightened the valves to the right place we also have replaced the whole cylinder head with a new one also i have replaced the valves and the carb has been cleaned what could be the source of the problem that not even mechanics can fix it? should i try and replace the oil filter that is pretty much the only thing i have not replaced yet on the dirt bike? please help there has to be someone out there who can fix this dirt bike??

3 things for a gas engine to run are:
1. Enough compression
2. The correct amount of fuel/air ratio
3. Strong spark at the correct time
If you have these 3 things an engine will run. Always start with the easiest/simplest first.
Check for these 3 things next time bike will not run.

Could be a bad coil.
When a bad coil gets hot, the inner windings expand an seperate.
When it cools down the windings contract and re-attach to each other.
Keep a known working plug with you.
When the engine dies - imediately check for spark.
Pull off the spark plug cap - insert the good plug - lay it's threads against the engine and try to start it.
No spark - check the coil.

I think its your electrical. You have a CDI box somewhere. Most are under the tank or the front number plate. Have a LICINCED mechanic take a look at your wireing harness and CDI box

sounds like the ignition, check the coil/cdi