AI7 not rebooting after shutdown.

My rig AI7 2.4 [email protected] 1.52V stock cooling with AS5 (open case). For a while I had no problems. Now it does the following: after long use (partially gaming) I shut it down normally and the next morning at boot up it beeps with checksum error and I have to clear the cmos and do the ritual of resetting everything.
What can cause that? My theory in the moment is heat on the north bridge or somewhere else after shut down has the same effect as running it too hot.
The only thing different from before is I have a fortron 550 installed (should be no problem) and got a Radeon 9800 pro (slightly over clocked 395/375) which creates quite a bit more heat then my geforce 2. I checked the temp last time before shutting down and it was 38 cpu 38 pwm and 31 sys, which is about 2 degrees more then before but not hot.
I am in the process of ordering a good CPU cooler and am pondering between the sp 94 and the zalman 7000. But if I have to cool the north bridge different, I am probably in trouble with those.

Corrupt BIOS and memory problems could cause it. I would first try re-flashing your BIOS. 1.6 Beta is the latest if you aren't already running it. If that doesn't fix it, run Memtest86+ to check your memory to see if you're getting any errors.

Your temps at shutdown look fine to me. I doubt that's the problem. As for a CPU cooler, I have no problems recommending the SP-94. I have taken a look and I do believe the Swiftech NB cooler will fit alongside the SP-94 if you want to run both. Currently, I'm using the stock NB HSF with AS5 in place of the factory "gum."