Older Mac, Newer Processor, Which RAM?

I put a top of the line Xeon 5690 processor in my Mac Pro 2009, and it's been working great for the past year. But I have been having random Kernel Panics restarts and I am thinking it is the memory.


Should I purchase DDR3-1066 RAM for the year of the machine (2009-2010 4,1 flashed to 5,1)

Or should I purchase DDR3-1333 RAM for the later 5690 Processor?

Thanks much for this help!

-- I am looking to purchase from this site:
https://macramdirect.com/macpro.html#mp3