of course you have the SLI bridge on them, right? Umm...
check the temps on your cpu? maybe it's throttling?
what's the brand on the psu?
do you have the right drivers installed correctly? did you uninstall the old ones fully?
you don't have the cruddy 256mb 8800gt's right?
That's all I've got. I hope you get it figured out...
just so people know you dont really need to have the sli bridge installed, might reduce performance by a percent or two but the pci e bus has plenty of bandwidth for cards to communicate, some people even have problems with sli bridge so are forced to leave it off.