Why does my KDF11 cause spurious traps to 4??

Hello, I've got a 11/23+ in a BA23 and when running RT11 I get spurious and unexpected traps to 4. After while I noticed that it will do the same thing while running the extended memory test built into the boot ROMs. I pulled out all the cards except the CPU and the memory and it still does it, and after swapping memory with another QBUS system that's known to work I've pretty much come to the conclusion that it's not the memory. Bummer - looks like the KDF11 is at fault. I actually cobbled the system together myself from an assortment of cards and parts, so it's not unlikely that I did something wrong. Is there any obvious mistake or mis-setting of the KDF11 jumpers that would cause this, or is my CPU card hosed? Thanks, Bob Armstrong