PC shuts down

How well does the CPU fan work though? You may need to add some extra fans, especilly for a pentium ll chip with only one CPU Fan. Add at least one other case fan and see what happens.

I agree with Joe, I have had a similar problem in the past. But if you still have problems there is ocassionally a setting in the BIOS that will kill the system at a certain temp. This can often be disabled, but it's at your own risk.

Check your BIOS settings for a hardware monitor section. (it is sometimes part of power management)
See if it has a shut down temperature set, or a setting to shut down the system if the CPU fan speed drops.

If you only have the fan speed shutdown, you might want to replace the fan on the CPU.
Make sure that the heatsink is making proper contact with the CPU.
Sometimes, a sheet that protects the heat transfer tape, or coumpound gets left on, and the CPU overheats.

I recommend that you install a hardware monitor program, or a case temperature meter.
This could be a cheap indoor/outdoor thermometer with the probe safely mounted inside the case near the CPU, but not in the air flow of the fan.

Monitor the temperature inside the case.
If it stays below 120F, you have adequete cooling.

If the system shuts down at a much lower temperature, you have a hardware problem.
I would suspect the motherboard. Only because it has the most solder connections, and sees the most physical stress.

Chas

One other thing to consider is that during the warmer months, the power grids become over taxed. It is possible that you are experiencng brown outs. Power sags can cause some odd symtoms. Some key signs of frequent brown outs are dimming lights (obviously) and frequently having to replace incandescent bulbs.

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