when it is acting up, do you see the reset led on the cpu light up? if it is coming on intermittently after first power up, and you have replaced the power supply board, and the power checks are good, i would start with the output cards. It is possible that the cpu or the memory or timer boards are resetting the system. If you have another to swap with, try swapping cpu, memory, and timer board. thats a quick check, other wise move to the output cards. remember the output boards can also affect the data, and address lines just like the input cards. if you dont have spare output cards, remove the output card that has the call lights, on them if there are no other essential outputs on that card swap it with other cards, leaving the output card for lights etc out of the equation. I have seen the cpu do exactly what you are saying. It could also be possible for the top rack decoding cirucitry to be causing the problem, i would change this last. you could just replace the few ic chips you see in the back of the rack if they are in dip sockets, good luck

What is the voltage doing on the bottom rack "when the problem is occuring"? Doesn't mean much if it checks out ok, when the problem isn't exhibiting it's ugly head. Put the scope on it, see what voltage is coming off the backplane regulator "when the problem is occuring". If you don't have a scope, you could hang a light bulb on it, of appropriate voltage. It will catch your eye better than most meters, and so you can observe more than one thing at a time. I like the way a light bulb is visible in my peripheral vision, so I can watch a couple of things at once.

Thanks guys. I made it back over there for a couple of hours today. I changed the top rack and 2 output cards before time got away from me. Some of the new output cards had a jumper for +/- 50v, I'll have to figure out what I have to set the jumpers appropriately. So I'll be back tomorrow sometime in between calls and repairs.