Well.... the problem is that my tranny throw out 4th and 5th, but only happens after running 15 minutes over 70 mph.... i really don't know what it is because, i can shift 4th and 5th clutchless and the tranny don't grind or anything like that, so what could be the problem?

 

Check your shifter bushings. They may need to be replaced. If they do, get some new bushings from www.boogerbushings.com