Need help with new Xid error

I was moving the interface card for my D870 between x16 and x8 PCIe slots today to get an idea of how PCIe bandwidth affects performance of my CUDA code. After moving the card back to the x16 slot, my CUDA app hung and I got this Xid:

NVRM: Xid (0007:00): 6, PE007f

Fortunately, the code has run successfully several times since cycling power. Unfortunately, I can’t reproduce the problem by moving the card around.

What does this Xid mean? Is there a particular type of problem should I be looking for?