Problems with AM2+ CPU in AM2 motherboard

[sa230e]

So I got a hold of an old computer for free and and I wanted to turn it into a media centre PC for my living room so I'm upgrading it a bit.

One of the things it badly needed was a CPU. It came with a single core AMD Sempron 3200+ (1.8Ghz) and even at idle the CPU meter is pegged at 100%. So I ordered a Phenom X3 8650 to replace it.

The motherboard is a Gigabyte GA-M61PM-S2 (http://www.gigabyte.com/products/product-page.aspx?pid=2373#ov). I flashed the BIOS to the latest version (F9D) with no problems. According to the motherboard website, this motherboard supported the 8650 since version F3.

CPU Support list: http://www.gigabyte.com/support-downloads/cpu-support-popup.aspx?pid=2373 (It's near the bottom)

So I got the CPU yesterday and installed it today and on first power up, I got an endless loop of BIOS beeps. I turned it off and powered it up again and it booted right into windows. When I reset I get the beeps again. Reboot again, it works. Every other boot it beeps. I checked the RAM and it is seated properly.

My question is how do I figure out if it's a bad CPU or the motherboard just won't support it (despite explicitly saying it will)? Or is there some setting in the BIOS I have to adjust to make them play nice?

 

[sa230e]

I just got (what I think was) a temperature alarm. I've only had it on for maybe 15 to 20 minutes and the thing is running warm - about 45 degrees C but not burning hot. The heatsink is warm to the touch. I'm using the stock AMD heatsink with Arctic Silver 5 thermal paste. Now it only boots on maybe 1 in 10 tries.

EDIT:

Repasted the CPU. Same problem. Thought it might be drawing too much power (The Phenom has twice to 3x the TDP of the old Sempron) so I tried it with a beefier PSU. Same problem.

 

[sa230e]

Anybody have any insights on this one?