ABS light comes on after cruising for 40'-50'. What's wrong???

What is your question?

Hello all, Here's the issues. 1. Rough idle, only on warm up cycle, idle is ok when up to normal operating temp. The other issue is with the ABS system, here's how it goes: I start the car, no light. I drive slowly and come to a stop, at about 3-5 mph the brake pedal jumps as though the ABS is operating, then I come to a complete stop, still no ABS light. Once it get going around 25-30 mph, or about 50' the lamp will turn itself on without applying the brakes. Is there any way to test the ABS wheel sensors? Another questions is, how critical is it to use the exact sensor for the car, my parts guy required a VIN to get the correct front left wheel sensor, he told me there were two different options for the 420 sel. I grabbed one at a pick n pull off a 500 sec, front right. Different part number and different mounting point for wire routing, the ohm reading seems to be very close to the new left side unit i purchased (which may be the wrong part anyway). I've swapped out the relays at the top of the ABS pump, no good-still same problem. Any suggestions would be greatly appreciated, I'm tired of chasing my own tail. Oh, one more thing, I did replace the front brake pads and rotors about 6 months ago, the problem occurred a couple months after that service was done, maybe a correlation, maybe not. Just thought I'd mention it.