next time it happens check the calipers. if its the handbrake mech that has siezed, you'll see the cable is loose in the caliper arm, if its the cable thats siezed the cables will be tight on the caliper arm. I had this the other day, and it was the cables at fault rather than the calipers! Which was good becasue I had just spent a fortune on MK3 calipers and rebuilt them with all new seals from VW! My new handbrake cables have arrived so they'll be going on shortly.