Here's the latest on this: I should start by saying that the problem the two computers were having were different, however the error message was the same. On the lesser of the two machines I had a micro ATX board w/ onboard graphics (Biostar A760G M2+). It turns out that the onboard graphics were taking too much voltage for the the PCI-E slot to consistently provide voltage to the graphics card, even w/ the onboard graphics disabled in the BIOS. The answer to that one was to a.) buy a new motherboard, or b.) put the old Radeon X1550 vid card back in which is a very mild GPU. I chose "b".
The other machine was not hardware related, but program related. Apparently when trying to install the newer graphics card, the DirectX versions were incompatible with each other. The only for sure way to fix that problem was to completely wipe the HDD and start all over. No. WAAY too many programs and hardware connected to those programs for me to have any desire to do that. The other option was to put the old 320Mb 8800GTS back in and hope that it would run okay. It seemed to me that the latter of those was a good thing to try because changing hardware would be easier. I did that and the machine has been on and running for nearly 24 hours now w/o problem.
I should mention that in both cases I uninstalled all the drivers from the previous GPU's and applied the appropriate drivers for the GPU's that are now in each machine.
