Thank you for reply,
I do not believe it is an ignition problem: the vibration is present in a continuos wave; its amplitude varies with speed; it does not go away when coasting. The amplitude becomes first apparent at 2k RPM @ 50mph, then increases to a very noticeable shake, smoothing out after 80 mph, only to sharply increase again after 85mph.
The previous work included camshaft replacements, during which a timing chain was split & allowed to drop into the engine. Rotating the crankshaft during chain extraction attempts could have damaged the valves, however, the compression is OK & there's no increase in oil burn. Eventually the front of engine had to be opened & the timing chain was replaced. Transmission was replaced but not the torque converter. All three mounts & both flex couplings were replaced.