1990 Cadillac Deville Vibration

Cadillac DeVille 1990 130,000 miles scrupulous maintenance
Engine vibrates; frequency proportional to speed and accelerator depression; coast smooth.
Two episodes. First no trouble light. Resolved spontaneously. GM dealer tuned engine. Smooth. Checked compression one cylinder lo.
Several days later vibration as above this time with trouble light on and dealer verified vibration this time. Passenger can feel vibration.
Dealer could not identify a code. Suspect warning light intermittent. Checked more compressions. Several low.
No antecedent symptoms; no oil usage. No blue exhaust smoke. Pristine outside and interior.
GM dealer says we need new engine.
Hard to believe because sudden onset; it has come and gone and now come back. Interim smooth as silk.
Two perhaps unrelated observations:
Two weeks earlier someone removed gas tank cap. Purchased a new one promptly; could something have been added to tank?
In earlier years trouble light meant a new oxygen sensor was required. Although auto was running smoothly. Light only symptom.
Like the car.
Desperate for advice.