GE Washer spins too slowly

GE GTWN3000M1WS washer works properly for everything except that it spins slowly leaving clothes wet. It goes through wash cycling normally, fills tub with water, agitates properly, drains properly but spins very slowly at about 20-30 rpm. Belt is very clean, firm, not slipping and pulleys and tub turn freely and easily by hand. Lid switch works - stops agitating or spinning when lid is opened then resumes a few seconds after closing lid. Motor blink code indicates "normal running" - 0.5 second on / 0.5 second off for 10 flashes in 10 seconds. Tried to reset motor by unplugging for 1 minute, then open and close lid 6 times but it still spins slowly. Any suggestions?