You never know, it could be a faulty sensor on a wheel causing it to go nuts because it thinks the wheel is going at a different speed. I know there are special abs codes, you may want to look into having those read.
I've had the same problem in my '94 SHO. Sometimes while braking slowly the abs kicks in, and when it does, I apply very little pressure to the brake pedal and the car come to an abrupt stop. The pedal feels like there is alot of pressure until I let off the brakes, then the car resumes as normal. Sometimes the abs light comes on and goes off. Other times it stays on until I shut the car off. I haven't been able to figure this one out.