The ABS sensors are on each wheel. As Sean describes pull the wheels, locate the sensors and clean with brake cleaner.......they get coated in brake dust.
If that doesn't fix the issue then there's 3 relays to check (there's a fuse also but in this case the issue is intermittant, worth a quick check but unlikely). The easy relay is the one on the main fuse/relay panel in the passenger side footwell.
The next 2 relays are on the ABS system itself visible under the hood drivers side near the power steering reservoir. The easiest way to get at he se relays is to remove the drivers side front wheel and inner fender cover. Then you'll see a cover over the ABS system that when removed you can view the complete control system.
All 3 relays are expensive so use one of the few diag. tools to check the system...........I use Theo's PDT999.
