The 9800GT requires a minimum of 400 watts. If your PSU isn't performing properly or going bad, it could be causing these issues.
400w for just a card <_< that is 150w more than a GTX 470. Actually to run one card that psu is enough unless there is defect. However each card under max overclocking and load can only draw 105w on stock volts. For sli a 400w unit isn't going to cut it. The only way to make sure that it is not due to low voltage from the psu is to measure the boot up value (in bios) the voltage on the 12v source/rail and then check with in windows by the use of a utility. Any thing bellow 11.5v on the 12v rail warrants a upgrade or replacement as you go low stability gets worse. Also if possible test each card separately to see if there is any results if not try them in another machine as both could be duds.