OK I think I've solved the problem. After two bakeouts it was still frosting. On the SBIG yahoo group it was suggested to check the faceplate screws. So it did and they were fine. Then I remembered what you wrote Jase about problems with the sealing of the CCD and I remembered that there was another O ring (big one) under the face plate. So I removed the face plate and the O ring fell out with two big creases in it
When the plate had been put on originally the ring must have been dislodged and part of it was protruding inside the housing so when the plate went on it pinched the ring in two spots where it crossed the O ring groove. That meant I had about a 2 cm gap where there was no O ring sealing the chamber
I've reseated the O ring but only time will tell if I need a new one. I will be contacting SBIG though.
The bugger is now that I have a huge big mote on the inside of the glass cover that I didn't notice the first 2 times time I used the camera. I'm wondering if it may have been sucked in during the drawing in of outside air.
I know flats will take care of it but it is frustrating.