Powerbook hard shutdown after boot if it's hot

Hi,

I have a strange problem that my Powerbook shuts down hard
right after boot, if the machine is hot.
For example, if I compile a kernel and then want to reboot
into it, it will shutdown the machine right after userspace
is booted. Cooling down the machine will fix the issue.
I first thought this might be a problem with the adt746x
chip driver (thermostat). But this turns out to not be
the case. I disabled the module (put return 0; early into
the module_init function), but the problem still exists.
I'm not sure how to debug this, as I don't see what's
going on. I tried to boot with init=/bin/bash. It will
boot into the shell but shutdown hard after 1 or 2 seconds.
It seems like there is some timer hitting in right after
userspace is up and running (some workqueue?). Strange is
that it _only_ shuts down the machine when it's hot
and it _only_ does this right after boot. If it survived
the first few seconds after boot, it's rock-stable and
it won't show any problems when getting hot (will drive
the fans correctly, etc etc...)

Any idea how to debug this?

-- 
Greetings Michael.