95 Transport revs & surges

Ok, I've got a 95 Transport with the 3800 that is running ok but it seems to surge a little when it runs. It idles fine & doesn't miss but will occasionally buck like crazy at around 55 mph or so, if you back off the throttle it'll straighten up. The really curious part is if it's running in the drive in park, when you push on the throttle & slowly bring up the rpm's when it gets to 4000 rpm, it quits & falls back to about 3300 rpm, catches & goes back to 4000 rpm, dies again, catches & revs back up. It sounds like you're reving it up but in reality I'm holding the throttle steady. There's no codes showing. The oil pressure gauge is below zero so I pulled the wire to see if anything changed. The gauge is now pinned at the high end but no change in the surging. I've swapped out the following: fuel pump, EGR, MASS sensor, Idle Air & coils checked all the vacumn lines without anything changing. I Ideas ???? Thanks.

Ok,... when you push on the throttle & slowly bring up the rpm's when it gets to 4000 rpm, it quits & falls back to about 3300 rpm, catches & goes back to 4000 rpm, dies again, catches & revs back up. .
Thats the rev limiter; normal; it acts only in neutral or park, so one cannot throw the pistons out through the hood even if he tries......but this should not happen in other gears....unless the transmission encoder has gone awry....but then that should throw a code....
It seems that with time, in some cars, rev limiter will act at lower rpm that designed for....and I've no refinite explanation for that; but the problem with your oil pressure gauge makes me think that problem with ground and 12V bus corrosion (often at splices under floor carpet) might impair some reference voltages and impact on computer behaviour...but ????.....
www.avigex.ca/xport/dtc31.jpg