Anything above 90 C really isn't something you want to run constantly. The instability will come from excessively high clocks where the card simply says "no, can't do it".
Dangerously high temperatures won't plainly tell you "no, too high" which is why you want to monitor them. To be honest, 100 C is ridiculous. Actually, I believe that anything in the 90s or above is ridiculous. It is normal. My eVGA 6800 GT @ stock maxed at 91 C in BF2. I water cooled it (DD MAZE4), and it idles at around 41 C and doesn't exceed 43 C under load. It's actually idling at 32 C as we speak. That 91 C just simply wasn't safe when constantly used.
I'm not saying to go spend money on a WC rig where it is unnecessary (in this case for example).
If you're actually getting 100 C (assuming that the sensor works properly), then you may want to look into trying an aftermarket cooler.
What is the ambient temperature in the room and case?