Doh, I should have remembered that 6100s (and 610s etc) have dodgy power supplies. Installing a G3 card will only make it worse.

Check whether the fan is running freely. If it is rough, find a replacement. Install it using the existing wiring scheme which incorporates a thermistor that shuts down the psu when it gets hot -- that is what is causing your lockups.

Make sure that your 6100 has clean airflow underneath the case and to the rear. If necessary, put a couple of blocks of wood underneath the 6100 to provide a wider air gap.