You are likely experiencing these shutdowns due to the CPU overheating.

The idle temp you list, is what you should be seeing under normal load, according to AMD the max temperature for the 955 CPU is 55°C - 62°C.

Make sure that all fans (case and GPU) are running correctly.
If the graphics card is passively cooled, it very well might be, you could attach a fan yourself to improve on the cooling performance.