1999 Chevy Express Engine starts and revs up and down by it

I went out and bought a fuel pressure gauge and tested the fuel pressure and it was Ok, steady at 52, but the engine was steadier than it had been. At the same time, I bought a ignition tuneup set, dist. Cap, rotor, etc, and went to put on the cap and rotor to find that the screw holes for the cap had been busted on one side and stripped out on the other, making the cap sit high on the busted out side. That was the problem. Thanks though, I figured it was something to do with the fuel too, but glad the problem is found.

Creatively of course. Piece of coat hanger bent across the top attached to each side to hold it down tight enough so I could drive it to Marta inspection to get tags. Need to get that going as soon as possible. I guess I'm gonna get a new distributor and put it in. Just flippin the van for a little money though. Thing runs great as long as the motor is firing right, but a pool company trashed it inside, enough clay and dirt to grow corn in all over the floors, dash, you name it. Good work van if it's cleaned up.

I thought it was the distributor cap but it looks like something else.
It will crank up and run great for a while and then it will go crazy, the speedometer goes nuts, the anti-lock brake light comes on, and it will run in lunges and then seems like it has died but if I take it out of drive into neutral it's still running and runs Ok, put it back into drive it lunges for a while and repeats the same deal. Seems like the gear shifting affects the problem. If you put it in 1st gear, it seems to still change gearsand will lunge so bad that the drive shaft sounds like it's coming apart. A mechanic locally put in a speed sensor in the transmission, ran Ok for a while and then started the same problem again.

There isn't an engine service light or code at all.

What could cause this?