Hello, this is probably not exactly related to your problem, but I'm adding it here just in case it might be useful to you, or if someone searches for topics on overheating units at some point in the future.
About 6 months ago I had a 2011 reach 80+ degrees celcius in a forced air ventilated rack (no air conditioning). This was witnessed in winbox, under system/health. The other 2011 in the same rack was at around 40 degrees, and a couple of 750UPs that were in the same rack were at aroung 35-40 degrees. CPU load on the overheating unit was not very high, if I recall correctly it was between 15% and 30%. In summary, the one 2011 was the only unit with a high temp, and it was the first time that I had seen such a high temp. I placed my hand on it just to make sure it wasn't an erroneous temp reading, since I was in the building, and I can attest to the fact that it was REALLY hot. Another symptom was that port 6 stopped working (no link, no light, no nothing)... It was very strange.
At any rate, panic set in, so I did not document my exact steps, but can clearly remember that:
1) I shut down the unit, let it cool down,
2) changed the power supply from an 18V generic to a 24 V original,
3) Pulled an export
4) upgrated Router OS and firmware to the most current.
5) Reset the unit and ran the export, and added a few things manually that the export doesn't script out.
The unit has not overheated again, and port 6 came back to life immediately. Since then I check the temp every 30 minutes and log it if it is >=50 degrees, and I haven't had a single event logged. If I had to guess, I'd say the generic power supply caused it, in conjunction with some other noise in the line, a rare singlular event. Then again, for all I know, it was happenning on and off again for a long time, and I only stumbled upon it that time by chance, which if true, would be similar to what you are witnessing. Nevertheless, the 18v generic switching power supply that I pulled from the 2011 has been used on a non-essential 750 ever since, with no problems whatsoever. Extremely strange, but true.
At any rate, I hope that you or someone searching these forums one day finds this info usefull. Kind regards.