Well, different DC projects will stress a CPU in different ways. I had a machine awhile back (an old dualeron that has since died completely) that would run two SETI crunchers just fine 24/7, but died when I switched to DNet. Sounds like something on that CPU is running very near it's maximum and the D2OL client stresses that part just a little more than SETI does. I agree with Carrot39 that is anything is overclocked, try cutting back a little and see if it improves.
JHutch