Ram problems with late 2012 27" iMac

My late 2012 27 inch iMac works fine with the 2 factory installed 4 gb memory chips installed in the bottom and 2nd from top slots. However, when I added 2- 8 gb ram chips in the other two open slots I get startup errors, and frequent restarts that fail. If I take the 8 gig chips out it works fine. I've seen other forum members use this configuration successfully. I also tried different placement of the chips, but no improvement resulted. I thought about resetting the pram, but - other than that I'm out of ideas. Any suggestions?