I think I might know what's causing the problem...
I'm using ARP mains studs and the oil pump housing needs grinding to clear the stud adjacent to the pump on the rear main cap. I thought I had clearanced the housing sufficiently, using the pump I removed as a template to work to. Unfortunately, it's virtually impossible to get eyes on the stud/pump relationship with the engine in the car and working under the car on axle stands. I tried as best I could to inspect this area with an articulated inspection mirror and a bright light and I thought it looked OK. However, I now have a theory that the pump housing is hung-up ever so slightly on the stud as perhaps I didn't quite remove enough material from the housing when I clearanced it. This would cause the oil pump drive stub to be at a v.slight angle relative to the oil pump prime tool. Under load, this slight (probably only 1* or less) angle would show-up as vibration through the prime tool into the drill.
The only problem with this theory is I can't figure-out how that could cause the pump to feel very notchy/jerky when simply turning it slowly by hand with a screwdriver. Perhaps interference between the drive shaft collar and the oil pump drive stub, since the collar is a fairly snug fit? Although I can't think of anything (without the distributor installed) that would constrain the oil pump drive shaft in a dead straight line.
I'll drop the pan again and see what I can see (damn PITA).