92 Lumina APV gets to highways speed; then stalls

[creemos]

I recently purchased a Lumina Van (92) from a gentleman who was frustrated that it kept stalling on him! He went to four mechanics... obvisously, I inherited his problem. This is what it looks like:

Starts and idles fine.

Runs well in traffic, coming to a stop, slowing down and taking off.

Good acceleration to highway speed.

At highway speed (+55 or 60 mph) runs well for days and then...

periodically, engine suddenly bogs down and starts popping and backfiring if I even touch the gas pedal attempting to reaccelerate

If I take my foot off the accelerator; it will cruise at 20 to 30 mph all the way home.

Even in the midst of this scenario; can restart engine fine.

Next time I drive; acts as if it just came out of the showroom.

Advice?