Yep this is a good place to post, I doubt if it's caused by going over
to broad band... happens when cold could suggest a power supply fault.

As a test why not run it without the Ethernet card or modem this ought
point you in the right direction, a good clue may be had by going
immediately into bios and take a note of the voltages and temperatures
for five minutes or so... they should not be lower than about 700mV
(0.7V) as stated in the bios screen, mind you you can still have
problems with the power supply even if the voltages appeared to be
alright.

All you need for bios POST (Power On Self Test) or bios boot is just
the CPU, Graphic card and RAM inserted with just these you'll be able to
boot the bios and access the set up screen... so it could be any of
these as well, sometimes re-inserting the RAM and graphic card cures
many a fault but I would be surprised if these was the problem.

I have had funny things happen when the bios back up battery starts to
get low so may be it's well to bear this in mind, the battery usually
starts going down hill after about 3 or 4 years.

NOTE
If you remove this battery for more than about 2 minutes you'll have to
re-set the bios settings, there's usually a 'quick set' button to avoid
this... better leaving this as a very last option.

Davy