Power management should be set to "off".
Couple of questions. Is this a AGP card or PCI? How old is the rest of the computer?
Some possibilities .............
1. Not enough power getting to the video card. It runs but when you put a load on it there's not enough juice and it shuts down. Thats hard to believe with a 9550, but anything is possible, especially with an old power supply that never was highly rated to start with.
2. Bad connection in the video card slot. Take the card out and check the contacts for anything odd or any dust bunnies that might have gotten stuck there, then look in the slot itself and check for them also. Once you have done that, and you reinsert the card, MAKE SURE IT IS ALL THE WAY IN! That used to be a fairly common problem with video cards, not having it inserted 100% in the slot.
3. Bad video card. There could just be a problem with the card itself. A bad solder job, the 3D stuff kicks in, and something shorts when the extra current starts running.
I think your original "out of frequency" error was a driver problem or setting problem that you fixed already. This is something else.