How old are your ignition wires, coil wire and distributor parts?

Examine all these parts carefully - once I found a rip in the insulation of the coil wire in a 740 - the wire was ripped inside the clip that held the wire to the firewall - I think this prpbably happened when some work was done but the wire was not removed from the clip, thus it got torn in a hidden location.

When I buy a used car, I wash the ignition wires in soap and water - in one 740 there was a burnt spot in one of the plug wires the size of a pea - the wire broke into two pieces when I tried to remove it,

Do your wires have all their looms and clips designed to keep them apart - some of the clips are made of nylon and attached to the cam cover - these clips are often broken or missing entirely.

You might observe your engine running in the dark - a light spray of water may produce a light show.

As for vibrations at a certain RPM - this might be due ti misfires added to natural harmonica in the engine - even 1500 RPM is not very smooth for a 4 cylinder engine.

I also suspect that engine management with a manual transmission is different than that for an automatic - I read about this when someone wanted to use a computer from an automatic transmission car in a manual transmission car - the advice was not to do it!