depneding on where you live, there might be a Rollei repair service. Here in Canada it is Kindermann Canada. They do great work on Rollei- and Leica projectors.
Before sending out your projector, do the following:
Look in the manual, in the back is a trouble shooting guide, you may get lucky there.
The other things you can do, is checking out the cooling channels for dust and dirt or ather contamination. To do this it is best to remove the top cover, never try to poke into the projector from the side! And dont forgeet to disconnect from the power first! The next thing would be to check out the bulb, it might be the wrong type, especially if you bought the unit second hand.
Another cause could be the therm-switch itself. It may defective. Those switches are usually standard components and you may find them at electronics stores or on the internet.
You can also contact DHW and ask for assistance or even send the projector there for repair.