check you coil and stator. i had a problem like this on my 500 and when the coil would warm up it would start to die

Try a new plug - 

It has new gas and a new plug!
Got it started today for all of about 10 sec's and it died again.Checked the spark.Not sure if i grounded it right but it wasnt a blue spark.More of a red?

Have you cleaned the carb in it.Maybee its dirty and the fuel not their.When it died ,did it stop all at once or did it sputter  and fire then sputter again till it died out.Could be the carb broken reed or weak spark.I would clean the carb  and check the reeds.Also pull the stator cover and make sure the flywheel hasnt throwing the dial key that hold the flywheel in place.I bought a 87 250r .That died and it would try to start and run a little bit.Boy fought he blew it up and after half butt a top end job on it.Long story short the dail came out of the crankshaft and the pickup wasnt firing at Top dead  center.He could have saved his self alot of money and a bike if he would have check that. But you will  need a flywheel puller to take it off.Unless the key has came out .Atleast i was able to get mine off when this happen with out a puller.Hope this was helpful to you and if  you need any help just message me.