There are a few things that could cause it. Crud stuck in the brake line that could keep the piston from releasing. You could try bleeding that caliper well to see if that helps. The seals or piston in the caliper could be worn. Replacing the caliper, or rebuilding it, would be fix. The slides on the caliper, which the slide pins go through, could be sticking and need to be lubed up.
Another, possibility that occurs more often on cars slightly older than yours, is the rubber flex hose running to the caliper starts to break down internally. It then works similar to a check valve, allowing pressure to be applied, but only slowly allowing it to release. Replacing the hose is the only way to find out if that is the problem.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.