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.
