My 88 Astro transmission will only occasionally.

Have the governor checked before assuming you need a replacement

Roy

What color is the fluid now? Should be red. If it is grayish, this means that the clutches are wearing out and you'll soon need a rebuild. [Driving it in second can speed up the process]

have the shift solenoids checked at a shop. If they charge a lot for the service; compare the price of a 2-3 shift solenoid to the price of their service; it just may be cheaper to replace the solenoids yourself instead of having them checked by a shop.