advice / suggestion wanted PDP 11/40 only boots XXDP, not RT11

My PDP 11/40 suddenly lost it's ability to boot RL02 disks except the XXDP disk. I have two drives, both boot up an XXDP (I have more than one) just fine, but any formerly-working RT-11 (v 5, 5.1, 5.3) no longer boots, it just hangs. I have been troubleshooting, running the 11/40 XXDP tests, but they seem to just hang too. And I am not a big fan of XXDP anyway. I have cleaned the drive heads, the disks are ok. I can load BASIC just fine from PDPGUI "tape" through the serial card (M7800) Any off the cuff suggestions why this specific issue would arise? Power seems ok, looking for dumb reasons that I missed. I have swapped out CPU cards, does not make any difference. I am pretty sure it's a UNIBUS interference issue, at least that's my working theory. I suppose there may be an RT-11 boot instruction call or routine of the CPU that XXDP does not exercise explaining why XXDP boots. Maybe RAM locations. In the end I just have to work through everything, but I am open to suggestions to help cut down the time spent diagnosing the problem. Thanks in advance, I will be around tonight (East Coast USA Time) Bill