Have you removed and rebuilt the basic computer components OUTSIDE of the case?
Literally just the motherboard, CPU, 1 stick of ram, boot drive and on board graphics (if you don't have this attach your graphics card), and see how it behaves like that with the power cables plugged in. If it behaves normally then rebuild the PC until you eliminate the problem.
If the problem came around AFTER fitting two fans it seems entirely possible the process of fitting them has upset something be it a loose connection, faulty cable, or something similar.