ABS light

this week the light has been going off and on. when i am driving it comes on, when i turn off the van it doesnt come right on but after a few minutes it comes on. i have checked all the wheel sensors and get good readings, the ABS mod is located under the air filter, i checked the connectors all looks good. i also noticed when the ABS light comes on the van runs like shit meaning no pickup, stumbling on take off, when the light os off the van runs great. i cant figure it out. thanks for your help everybody.

One easy thing to check is the brake fluid level in the master cylinder.
The low fluid "trip point" is very close to the full line.
I just yesterday had to add some brake fluid to mine......when I would accellerate hard....or make a sharp turn....my brake light would come on......and if it was on long enough....the ABS light would come on....and STAY on until the vehicle was shut down.
Filling the master cylinder up to the full line solved the problem.
My brake pressure switch is still dry (a MUST check) and I will continue to monitor......to see if I have a leak.

In short......low brake fluid can cause the ABS light to come on....and it is the easiest thing to check......