Intermittant bootup

I have a HP Pavillion 743 that boots to XP home just fine 99% of the time and then for no apparent reason throws a message that it can not find a disk and to insert a boot disk.
It behaves as though it has a corrupt MBR but then later it will boot up just fine.
I used Micro 2000 testing software to run continous tests of the mobo, hd controller, memory and a read test on the hd which is an EIDE 80 gig. It shows a 55aa boot signature and it says the drive is bootable but it still won't boot up every ten days or so.
It is a relativley new surge suppressor.
Has anyone seen this? Battery Maybe?

Old Salt

 

I have seen them do that when the power cable to the hdd isn't plugged in securely. Maybe check both cables?
Install a new ribbon cable?