It is hard to come up with a reason the pump would work well at one RPM and not another, and if their was a problem here I would expect it to be OK at idle and progressively worse at speed. But just to be sure, Cees are you absolutely sure the pump collar's timing mark was dead on when you reassembled? Was the rebuilt pump exactly the same as the old one (the timing for older pumps is very different than for newer ones, but reading your posts I assume you are already all over that possibility)?
LiekLb, I was more inclined to worry about a worn or non-functioning mechanical advance in the distributor, which would cause poor mid-range performance when vacuum was not getting the timing advance you need, this problem would also show up as a progressive (over time) issue. I understand the Doc has rebuilt a million of these and may offer some advice. The other real puzzle is the backfire on liftoff; that would tell you the manifold was getting fuel somehow, which I think is a telltale: I can only induce a "burble" or very slight backfire if I slightly depress the throttle when the car is in negative acceleration and in gear. Have you checked the throttle damper?
Sorry for the ramble, but this is an interesting challenge I hope you can resolve and disclose. It flies in the face of the medical addage that maintains that complex, seemingly multi-variate problems usually have a single root: parsimony is the word for the day.
Best,
g
'64 230sl, fully sorted out...ooops, spoke too soon
