Well, I don't have much more to suggest actually. An intermittent problem like that is probably going to be something sticky, like the EGR valve (which you replaced) or advance weights. (also SHOULD be fixed apparently)
If the throttle plate bushings are shot, the idle RPM should not be consistent...few hundred RPM difference every time.
Since it is intermittent, it's not that something isn't adjusted right. Have you tried disconnecting the vacuum lines (and plugging them of course) for the EGR and vacuum advance, and testing it?
I'd ignore the stock timing specs, and advance it until it pings under load, then back off a few degrees, making sure it still turns over easily. Not ideal method for timing, however, it still sounds like spark is occurring at the wrong time.
