If the ventilator (you mean cooling-fan?) is changing speeds then its PROBABLY the PSU being too weak.
OR one the the additional connectors is intermittant bad.

Thats by best 'educated' guess.

But I would first try testing it in YOUR PC, but on your FRIENDs monitor,
or test it in your FRIENDs PC, using YOUR monitor.
Just in case its your monitor at fault, or the bale between your PC and monitor.