WEEEE! Thank god someone else is having mysteriously high temps and it's just not me! (Sorry) My deal is about the same... High ambients 85-90.5 and 40C idles... then magically with the A/C on PERMAFROST mode: 21.5C ambient and STILL 40C idles... pure impossible! I'm thinking you, like myself, might have a screwy internal sensor...
Of which I don't know where it's located on my mobo A7N266-C (POS!)...
Sorry, this didn't help much, but the senor reading could be misleading...
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XP 1600+ AGOIA-Y (GreenLabel, UNLOCKED!@/ NF7-S 2.0/ PC 2700 DDR
10.5*172FSB(1806/ 1991 high), Vcore 1.792...@84.95Watts
--86' Chevette Heater Core
--1/2" Vinyl tubes
--Via1300-Stealth Mod(QUIET!)
---Home made copper-tube block
---PVC small internal res