Transsmission wont shift when hot

I have a 88 chev 1500 2wd 4.3 with a 3 speed auto THM400 transmission. After about 30min of driving it wont shift from 2nd to 3rd it just revs up like its sliping if you keep it under 40km/h you can drive fine but when it tries to shift it just revs. If you let it cool down for 10 min it will be fine again for 30min or so. I changed fluid and filter, also put some lucus trans fix in it but still does the same thing, help please, i dont want to have to change this transmission any ideas ??

First of all you probably dont have a THM400. That trans is a heavy duty 3 speed usually only on the back of 454CI engines. Anyway, sounds like you have a valve sticking, if it shifts good when cold. You can try rebuilding the valve body. Its can be difficult if your not used to doing things like that. Otherwise you can get a reman valve body from a local parts store. Last couple that i bought ran no more that $80 but dont quote me.

Does reverse work?

ya reverse works fine, and it is a thm400 they were in a lot of the 88 and 89 chev 4x2 with 4.3s not just 454

anyother ideas ?