... and now it's idling at 45 degrees and getting to over 70 degrees full load, while my heatsink and fans are completely untouched. What happened? I just put new thermal paste on the heatsink - is it that maybe air bubbles are forming in the paste or something? This inexplicable phantom heat is really getting on my nerves.

Probably to much paste, or applied incorrectly.  What heat sink is it, and how did you apply it?.

Check for dust in and around the case. My GPU normally idles at 45 degrees but I hadn't cleaned out the case fans in a couple months. The other day I checked and they were at 68 idle, went back to 45 after clearing out my dust filters. 

"Probably to much paste, or applied incorrectly.  What heat sink is it, and how did you apply it?. "

" @Korwin said:

"Probably to much paste, or applied incorrectly.  What heat sink is it, and how did you apply it?. "

Yeah, probably this. "

it's not dust. check your usage during peak temps. check your temps manually, electronic motherboard sensors are notoriously unreliable.

Pretty sure mobo temp sensor's are a thing of the past, most cores have internal probes these days.  That being said you can get some pretty dodge probes, my Q9550 always registered a 13 degree variation between core0 and core3.
 
If you want to check how active your probes are grab a copy of Real Temp (it's free), it has a probe test as part of the application.

"@cspiffo said:

" @Korwin said:

"Probably to much paste, or applied incorrectly.  What heat sink is it, and how did you apply it?. "

Yeah, probably this. "

It wasn't a lot of paste, and the computer was idling at 35 degrees AFTER applying the paste. It's just shot up for no apparent reason over time. Could that much dust collect in a week?  Edit: Wow, now it's idling at 35 degrees again, and reaching a max temp of 60 degrees. Could it be that my chipset is reporting incorrect temperatures? "

Cpu tempertures can get very high depending on the actual cpu, that might be normal. You should google what the average temp on load is, I think that's normal because you haven't really posted or checked the temperture before. Obviously posting which cpu you have would definently help.

Another thing worth noting is a large number of thermal pastes have a "set in period" where the paste slowly evens out and firms up.  It's quite common with a lot of silver based pastes like AS-5.  Pastes like MX-3 don't have a set in period and thus you get immediate full thermal benefit.

Run a prime95 small FFT test and watch your temps w/ coretemp.  It's possible the hsf is not properly mounted -- pins sometimes are not all the way in or it's not tight enough.

Now my CPU is idling at 32 degrees, once I overclock again it will probably still idle under 40. So whatever.

Update - so I plugged a big hole in my computer case where my damaged DVD drive was missing with the fixed DVD drive, and the CPU is idling at 27 degrees and reaching only 52 degrees full load. I didn't realize air flow could affect temperature by that much.