Is this a new build? Or was it working before, and suddenly stopped working?
If replacing the video card didn't solve the issue, I'd start looking at RAM & CPU next. If you have multiple sticks/dimms of RAM, I'd try taking some out. Possibly try turning it on with only a single stick of RAM in it. Then try the other stick.
If that doesn't work, it could be a faulty CPU or motherboard.
If this is a new build, or possibly even if it's an old build, you might want to check if the heatsink is seated and secured properly. If it's a new build you'll also want to check to make sure that if there was a protective plastic film over the contact surface of the heatsink, you removed it before securing it.
It certainly sounds as if it could be heat related, thus the advice.