The Gas Gas 300 is as reliable as they come. I have yet to see a jap as bullet proof as the Gas Gas (besides the PW50
). I've known GG owners who tear down the top end out of guilt more so than from any kind of mechanical failure. Usually what they find is a set of worn rings, a pretty, perfect piston and a cylinder that just needs a little scotchbrite. I'm talking moderate-hard raced '01 and '02 models here.
The biggest problem I've had with my '03 200 is that I can't seem to stop adding goodies to the bike, otherwise it's the most reliable bike I've ever owned.
I'm just guessing you're jetted lean or light on your premix ratio. Can you give us some insight on your jetting settings, premix, altitude and air temps? Maybe we can get you on a baseline to help hunt down your serial seizure problem.
Get her replated, pick up a new piston and rings from GoFasters. Find a baseline jetting setting (there's tons of jetting info on this site) for your conditions and let's see what happens next.