Another thing that can cause the abs light to come on is dirty brake fluid or old brake fluid that has too high a water content. The water content problem is what caused my old '92 to have the exact same symptoms you are starting to see. The light would pop up, but disappear if I turned off the ignition and restarted. It got more and more frequent until it stayed on all the time. When we pulled the codes, it said 'low fluid pressure' but a mechanical gauge said it was fine. We purged the system and the problem vanished permanantly. Brake fluid is hydroscopic. After it's been in the vehicle a long time, it absorbs enough water that the specific gravity of the fluid drops, and the sensors think the pressure has gone down. Getting a fluid flush costs about $25 from a shop and is worth the $$. After we flushed mine, it took about a week for the symptoms to disappear, since the abs valve is on a dead end of the system, and the new fluid had to work it's way in.
