[Kymmy]

If the fan is speeding up and slowing down it sounds as though the hardwired POST test is resetting the machine. Normally indicitive of a hardware fault. Strip the PC down to it's bare essentials needed for completing the POST (PSU, M/B, CPU/FAN, MEM, VID) If still no luck then if you do have a speaker try reducing it even more (remove the VID and MEM) and see where the POST error beeps lead you. If you get beeps indicating MEM error then add the MEM and see if you get the next beeps in line. Without a POST indicator card (or built in LEDS) it's the ONLY way of diagnosing boot errors pre-bios screen without using spare hardware. When you eventually add the bit of hardware that duplicates the fault you've found the problem.

One thing to check if you have a meter is also the voltage rails (or swap out another spare if you have one