Cross-posted I guess? (or I wasn't thinking about the whole story?)
As Bernard suggested, do check out the main case cooling fans. If needed, replacements shouldn't run over $20 (US) each, and with recent case designs they're pretty much a "snap in" replacement. With older cases you might have to actually turn about 4 screws.
Case fan failures usually produce other symptoms, but they might be involved and should be checked occasionally just on principle. Note that in some newer designs it's normal for the case fans to run only intermittently. Usually one that's gone bad will be full of junk, stiff to turn by hand, or otherwise obviously degenerate.