97 Astro automatic transmission intermittent problem

My 97 Astro has an intermittent shifting problem after it has been driven at highway speeds for some time. Around town and short trips it is fine. The problem is that it shifts hard from 1st to second and sometimes from second to third. After it sits for awhile it seems to be okay but will occur again after driving for long periods down the freeway. What is the likely problem and will it be an expensive repair. Are there additives that will help the problem? Hi: Based on what you are saying, My guess either your convertor is not locking up corectly or your intermeadiate clutches in your trans are wearing out.

How many miles on your Astro?

You could try a transmission additive to see if this could help your problem. They are not that expensive Car Repair Talk's forum.