Yeah well, there's a good chance that it is due the cpu.
I've seen it more often with crashes.
Repairing is often not possible and mostly more expensive than buying a new box.

Buying a new motherboard is a option, but the prices are not so interesting to buy it.
You could think of around the 80 euro's.

I would just buy a new 800se box, which depends on what you want.
A Sunray SR4 v2 box with tripple tuner I've seen for around the 130 euro's. while a normal 800se Sunray box cost around the 113 euro.