400/500 series cards that are overclocked by the manufacturer are likely the blame there, msi/gigabyte sold a heap of 400 cards that leaked tdp like crazy and artifacted at high temps because of it.
I've had several 600 gpu's at 85c in tombraider/folding and not had them drop back to 732, the lowest they got was to the base clock.My comments on throttling are also taken from personal experience when related to the 600-700 series cards.
You are right about the temperatures for the 600 and 700 series cards being higher (70C and 80C respectively), I was simply trying to recall from memory when I made the post. I'll edit it to reflect the correct information.
My personal observations include a GTX 670 and GTX 780 dropping well below their "base clock" when over 80C with the lowest clock at 732mhz where it no longer dropped.
there were driver issues that were causing it to get stuck in low power 3D,
A cpu throttling due to heat will reduce gpu load, and thus result in the clock rate falling back as well.
MSI (and others) were caught using way to much voltage on those gpus, which increased heat, increased tdp leakage and thus increased instability of the transistor when under load. they were again caught doing so on 600 series cardsAs far as the 500 series goes... I have gone through about 4 different GTX 570s which all started to artifact at above 80C and one's transistors even melted at a sustained 93C (don't buy MSI...).
I see reports of this, but have never experienced it myself - even though I'm rather sensitive to input, audio and visual lag.The biggest problem I have with the 600 and 700 series cards IS the GPU Boost 1.0/2.0 as it causes some "frame rating issues" (smooth frame rate vs. number of frames per second) when it kicks the Boost in. To combat this, I simply overclock it so that it never needs to Boost, but I also needed to install additional cooling so that it would never throttle the speed down above a certain temperature.



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