startup shutdown prob.

I'd say make sure that your CPU and RAM are seated completely. I've seen this happen when I've mounted a CPU heatsink wrong, causing some of the pins to be just barely in contact with the socket; when the CPU warmed up it would shut down, probably because the heat expansion caused a disconnect. Similar with RAM, if the pins aren't snugly in the slot it might cause intermittent probs like this.

Also you could try going into BIOS and toning down settings related to memory. Not sure what hardware you're using; the Dell Phoenix BIOS tends to be very terse as far as features they let you modify, but 3rd party motherboard makers like ABit give you a slew of parameters you can mess with; if this is the case, set it to "Failsafe" level settings and see if that changes things, then slowly increase parms and see if you encounter more probs.