When im driving my 87 IROC z-28 with 5.7L 700R4 transmission it will not go into third gear around 60-70 mph. it just hangs right at about 3000-3500 rpm's. it goes into 2nd fine but just does not go into third. What could be wrong because it done this a few months ago and we replaced the fluid and filter and everything run fine. That was not 2 long ago and i think something else might be wrong with it

 

I had the same problem in my car about 2 months ago I took it in to get it rebuilt and the guy said "you burnt up the third gear clucthes" anyways I got it rebuilt and havent had any other problems out of it.
Bryan