If you've had the GFX card for 3 years, I would seriously clean it for dust or something (unless you've already done that)
Anyhow, the symptoms you describe sound like the GPU is overheating. You get the same result if you overclock it without proper cooling.
And another thing - motherboard temperature sensors should never be trusted, as they're not calibrated. They can show you a range, say it gets 10 degrees celcius hotter from idle to load, but thats about it. Motherboard makers just slap the chips on the board and ship them out without calibrating, and that also means that you can have totally different temperatures than other people, so its no use comparing.
Anyhow, the symptoms you describe sound like the GPU is overheating. You get the same result if you overclock it without proper cooling.
And another thing - motherboard temperature sensors should never be trusted, as they're not calibrated. They can show you a range, say it gets 10 degrees celcius hotter from idle to load, but thats about it. Motherboard makers just slap the chips on the board and ship them out without calibrating, and that also means that you can have totally different temperatures than other people, so its no use comparing.