87 astro transmission

hi guys, im kinda new to this so please work with me. i have a 87 chevy astro, fopund it on craigslist for $300,(the a/c blew cold, so i was sold). i have had it for goin on 8 months now, have serviced it like no tomorrow.but 2 nights ago, i was around town and was accelerating from a red light.went through first gear fine, went into second gear fine, then the over rev began.me scratching my head was quite confused because 45 seconds before we were running fine and slowing down for a light.i pulled over and started again, 1st was fine, 2nd was fine, around 30/35 mph when it should be about ready to shift to 3rd, the whine again. i parked it, called my brother, and we went to work, pulled the pan, drained the fluid, changed the filter, filled it back up and let it sit. went back today to get it to bring it home and crunk it up, went down the road, and the same thing happened, so i manually had to drop it into second and limp it home. does anybody have a clue at what the possible problem could be, i was thinking maybe a modulator, or stuck valve body, but this is my main transpo, so i have to figure something out quick, thanks for your time.

Not likely a modulator on the 700-R4. It's possible the direct clutch (3-4) is slipping or not getting adequate fluid pressure. That's not uncommon on this trans, and is one of the weak points which would normally be addressed in an overhaul.

Short of removing the trans, you CAN remove the pan and filter, clean the area under the vehicle, and remove the lower valve body and separator plate. Be aware that there are several check balls in either side of the separator plate, and they all need to be installed in their proper positions upon assembly. The separator plate may have enough wear to have allowed a check ball to slip through. Separator plates with this problem can be replaced or repaired. There are kits available to install a hardened seat into the plate.

As you mentioned, the other possibility is a sticking spool, but that will be evident in the removed lower valve body.

Have you tried to manually upshift through the ranges to third?