1999 Chevy Express 1999 Chevy Express Van Engine Problem

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 gears and 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?