1997 Lumina surges @ 50 MPH

I have a 1997 Lumina 3.1 with 100,000 miles on it. When I get going around 50MPH the rpm's drop and the car surges and acts like it about to die. As soon as I give it gas it clears up and drives fine. This has happened about 3-4 times in the last two months.
No SES light, or codes,
Tried replacing Fuel Filter, Plugs and plug wires.

Any other suggestions?

Sorry if this is a repost, I thought I saw something about a month ago, but can't find it now. Maybe it was on a different sight.

thanks

Do a full scan while under these conditions ( 2 people) try and do a freeze frame when it occurs, migh also check fuel pressure.

It only happens once in a great while, like every couple of weeks, nothing consistant. Also forgot to mention that it only happens when cruising steadily around 50 MPH for a while. If I am accelerating I can go right up to 70 MPH without a problem.