Not expert but my 540 does the same for first few secconds after cold start, think might be combination of moisture on ignition system (ht leads etc) as well as minor timing problem (due to clearances not spot on because of age, exhaust not standard? etc.) This combined with old engine mountings causes shakes, cause everytime motor misses beat motor shakes.
That is where i think my V8's shake problem, comes from when cold, don't know.
But goes away after 30 sec?