Mac randomly shutting down

for the past couple of weeks or so...my mac randomly decides it wants to go into sleep mode or something, im really not sure what the problem is, but it seems that right before it does the fan speeds get really high. is it overheating or something? i dont know what to do and i dont really want to take it in and have to pay a buttload of money for no reason.

i have a G5
Processor: 1.9 GHz PowerPC
Memory: 1.5 GB DDR2 SDRAM

See if this helps.

Resetting Cuda/PMU on Power Mac G5, Power Mac G4, Power Macintosh G3