Try turning off the GUI client, and running two copies of the CLI version. In 6-7 hours on your QS, both should finish up a work unit each.
If they seem to be running slow, open a 3rd terminal window, and type "top" followed by Return. You will get a process listing, including the percentage of CPU each process is using. The two SETI processes should be about even at 40-48 percent each.
Kernel panics are usually caused by unsupported hardware issues (usb devices, sometimes), or hardware faults. If you really suspect a hardware fault, run your Hardware Test CD on your unit. To test for unsupported hardware, try pulling 3rd party PCI cards and peripherals. Run without them for a day or two, see if the panics go away. If they do, you will have to try a few 3rd party addons at a time, until you find the problem one(s).