sometimes my monitor comes up saying 'no signal detected' and goes black
When and how often? Once a week? Several times a day? Immediately after boot after a full cool down, or only after it has fully warmed up? When did the problem start?
While I agree this looks heat related, I never like to move on with intermittent problems without knowing I have a good power source first. I would swap in a known good PSU
to be sure. This assumes, as Matt suggested, the interior has been cleaned of heat trapping dust, all fans spin freely, and all cables have been checked to ensure securely attached.
I would also advice you don't focus entirely on the graphics card as a failing or overheated card does not normally cause the monitor to go black - but instead the image will become pixelated or frozen, or wrong colors. This might not be the card.
You might try blasting a desk fan inside the open case and see if it happens. What are your temperatures?
Tell us about the computer and card?
Inspect the motherboard for bulging or leaking electrolytic capacitors. These failed or failing capacitors are a common cause of sudden, but seemingly random system lock ups
and reboots. The capacitors look like tall soda cans, many of which surround the CPU
All older motherboards, and many of today's less expensive motherboards use electrolytic capacitors containing a liquid
electrolyte. Failing (including flawed and/or abused/over-heated) capacitors literally bulge at the seams due to excessive internal pressures. Extreme (and very rare) cases result in a firecracker type explosion that can really stink up a room. Typically, electrolyte just oozes
from the pressure relief point, which appears as a symbol or letter stamped in the top of the capacitor casing. The electrolyte can be caustic to motherboards and flesh. Look for white to dark-brown, dried liquid or foam on the tops or bottoms of the capacitors. Bulging capacitors are a sign leakage is about to occur.
A motherboard with bulging or leaky capacitors can be repaired, but often it is more cost effective in the long run to replace the motherboard.
Be sure to first
power down, unplug
the computer, and keep yourself discharged by touching the bare metal of the case before
I would test your RAM
using one of the following programs. Both require you to create and boot to a bootable floppy disk or CD to run the diagnostics. Using the floppy method is generally easier (and another reason to include floppy drives in new builds). However, the CD method is just as effective at detecting RAM
problems. Allow the diagnostics to run for several passes or even overnight. You should have no
Windows Memory Diagnostic - see the easy to follow instructions under Quick Start Information.
MemTest86+ (for more advanced users) - an excellent how-to guide is available here.