Mac Pro 5,1 lost video

I have a 2010 mac pro dual CPU, 2 x 6 core Xeon X5650, 128 GB memory distributed across the 8 slots. I purchased this computer a little over a year ago, and installed a Nvidia Quadro K5000. I used OpenCore Legacy Patcher to upgrade to Monterey about a month ago. It was all running fine for the past year, including following the upgrade of the OS. About one week ago, the graphics became scrambled. I switched to the other port on the graphics card with return of the normal picture. A few days ago I lost any video. I tried swapping to some older video cards that I had available. With a couple, video returned for about 30 seconds, then disappeared. I’ve repeatedly reset the PRAM and SMC, swapped the processors into the A tray, removed all the storage units and other PCI cards, removed all but two of the memory cards and swapped them around, all to no avail. I suspected that one of the cpu’s might have failed, and have ordered replacements. But that doesn’t seem to be the issue, as trying to run with either processor alone in the A socket hasn’t helped. Now, I’m wondering if the problem might be in the processor tray, or the back board.
Any recommendations?