I'm just going to throw this out there because only Ford seem to have this problem, thats not a Ford dig by the way.
I think what you may be experiencing is valve guide bind, the only inconsistant part is the rollers and that could be due to subtle throttle fluctuations, when it cleans out, does it stay clean? and how consistant is that 3500 rpm ceiling.
The other possibility is a bad ignition switch or electrical to the ignition system, like a fuse holder or terminal causing a voltage drop to the box, at certain RPM could vibrate at that perfect frequeny. To rule this out just use a jumper for a quik run and see if the problem goes away.
Sorry to hear about the problems, this kind can be very frustrating.
BTW if I was going to do a Ford dig, it would be "hey the distributor is in the wrong end of the engine!"
Good luck!