I had a similar problem on a 2000 G-SR sedan, my ABS light began coming on randomly, immediately after I changed the brake pads myself.

I figured I messed something up permanently... BUT when I changed my brakes the next time, I discovered that one of the metal backing plates which slide behind the brake pad during installation had slipped out of its correct spot (presumably during my installation).

When I put the new pads on i took special care to watch these backing plates, and a short time after the installation (not immediately) the ABS light went out for good.

DOnt know for sure that this was the problem... but it was an extremely suspicious circumstance.