BIOS CPU Temp 88-90C, but Easy Tune and Core Temp in windows are reporting ~45C
Both BIOS and Windows suggest the System Temperature is around 40C, no problem there. Using stock CPU cooler, which is affixed correctly and is plugged in and the fan is spinning etc. etc. no visual problems.
I found a digital thermometer for human use, as it doesnt display over 42C, but couldnt rech the CPU when the heatsink was on, so what i did was, I left the PC running for 5mins, the BIOS was saying 88C, i turned it off, the second i turned it off, i quickly unplugged the CPU heatsink/fan, which i could easily touch with my fingers instantly, didnt seem like ~90C, then i touched the cpu top layering itself and could keep my fingertips on it for about 5 seconds before it became to hot to touch. Before I did this, i quickly touched the CPU with the thermometer and it read as 39C, i then tried turning on the PC without heatsink affixed and the thermometer gave a reading of HI (or no display) as the temp was obviously above 42C as it does not read above that. I quickly went into the BIOS and it said the temperature was 89C and it stayed there, I did not fluctuate at all, although the System Temperature temperature was fluctuating 1-2C give or take. The comp eventually shut off after about 30 seconds....
I'm guessing you're awaiting a (yet another? ) BIOS fix; my first though was a new stepping, but there's only been one core (B1 - CPUID 106E5h) for the 860 so far... Welcome to the 'new adopter's blues'! I wouldn't worry about it so long as a few other sources are giving you reasonable temps... Note that the BIOS releases for this thing have averaged one every eighteen days - there's a reason for this!!
Yep, I had the opposite issue with my PII 940 on a Gigabyte board. In its case it read 25C too low (and would turn the CPU fan off!). Sure enough though, as bilbat said, two or three BIOSes later fixed it. You could try sending an email to Gigabyte tech, just to make sure they know (or if they have a beta fix).