Monitor says no signal but PC is running
So I've just had my power supply replaced and my computer starts up, the operating system boots up and I had a video signal but everything was running really slowly then my monitor just went black and says no signal. I've changed monitors so it's not that, I've changed hdmi cables so it's not that either. The graphics card is set up properly and and everything is connected correctly.
I'll get a video signal for a couple minutes but as soon as programs start booting up the signal goes.
I have no idea what could be wrong and don't really want to take it back to the repair shop cause they kind of took the p*ss. Please help I've not been able to use my computer for 2 weeks now and I just want it to work.