PDP-8/A hardware weirdness

I'm having really bizarre hardware problems with my 8/A (M8315/6/7, 32K, programmer?s panel, limited function panel, two RL02?s, OS/8). Most of the time it works perfectly, including last night, but today it began refusing to boot OS/8 again. If I toggle in a short program (say, to read the keyboard and echo to the printer) it works. So far so good. BUT if I hit the boot key (which doesn't boot correctly, although it does access both RL02's briefly), then halt the system, then run the test program, I can see while single-stepping that after two instructions it jumps to location 0 instead and thereafter into space!! Even more oddly, halting the system again, then running the test program again, works normally without the unwanted jump to location 0... every time. Until I hit BOOT, halt, and try and run it, then it jumps away again... The only thing that occurs to me right away is that power-fail & restart has an option to jump to address zero. But I've got that option turned off. Time to get out the logic analyer and scope... unless this weirdness is something you've heard of before? Any hints appreciated! thanks Charles