I've got the same issue. Today I have changed my gearbox thermostat (£11 for a Febi one from eBay - Parts in Motion - part code 1K0121113A, or Febi 172376) after reading some posts on here. Had the car running for half an hour on the driveway, and it got to 70 degrees. Took it for a thrash about, and got it up to 90 degrees, so thought the problem was sorted. Parked it on the drive while running for another 20 minutes while I washed it, and it went back down to 70 degrees again.
So I'm guessing with mine it's the main engine thermostat that's the problem, or the radiator temp sensor? Just about to do some research on here to find out which one to tackle next!