heaters work fine (nice and hot) until the engine temp hits 90degrees then they go warm i am thinking its a thermostat problem has anyone had a similar prob and how did they fix it 
its unlikely to be your thermostat because your car is heating up fine, but does your car maintain a constant temp on the temp gauge, if so then you can rule out the thermostat..
One thing everyone here at this forum will advise you is that if having a problem with your heating change the "BY-PASS VALVE".... its only 28 squid inc VAT.....before you take my advice see what the gti geeks say