Ian,
You should first look into the simple adjustment of the TV cable. It is on the carburetor and can cause this problem.
That being said, the 3/4 clutch could be burning up and if that is the case, you are looking at a rebuild.
Check out [http://www.hotrodders.com/forum/89-700r4-tranny-slipping-65593.html|this thread] from another question for more info.
Mine did the same thing in my 86 K20. It started with 4th going out, then i lost third, deteriorated further to losing 2nd. And then reverse. It took almost a year for all that.
The good news is, there is tons of info and plenty of parts.
Regards,
Frank