Hi, pittbull:
An intermittent that happen every few hours will be a pain to find. I would try to find the source of the glitch, and attack it at the source.
What is the source of the clock? Could some event be causing it to glitch? I have found these problems hard to deal with without using ~SlaveSelect, but easy with ~SlaveSelect.
Nowadays, whenever I use the SPI in slave mode, I bring ~SS high when the SPI is idle. That will re-synchronize the SPI clock if it does get in trouble. If the glitch occurs outside of a transfer, it will be ignored if ~SS is high.
Hope it helps . . .