Gilly is on the money with the guage, however maybe one of the temp sensors is faulty which is telling the guage that the car isn't warm. To test this you should find a large hill and chuck the air con on and run it hard up the hill. See what the guage does. If it's warm outside you could hit 105 mebe 110 so compare what you get. If the guage doesn't move much then I would say that a sensor or something is faulty as your car must heat up with a run like that. If the cause of your trans problems is the modulator do a search here and you will find the instructions on how to adjust the modulator.

Matt.