My 1991 560SEL idles rough, but smoothes out during acceleration. I replaced rotor, cap, wires and plugs but I still get severe idle. The transmission shifts sporadically. Dealer suspects leaking intake gasket causes both issues - incorrect oxygen mix and vacuum loss to engine and transmission. They want to replace the intake gasket ($1100), then test again. In case I don't feel like doing this expensive trial and error approach, can anyone point me in the right direction? Compression tests well in all cylinders.

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1991 560SEL (254K miles)
1980 Lincoln Mark VI - Givenchy
1979 Lincoln Mark V Collector's Series
1979 Lincoln Town Car Collector's Series
1979 Ford "Effie" Pickup (in gradual transformation to a show/street truck)
1969 Lincoln Mark III (waiting for "Overhaulin" to call)
1989 Lincoln Towncar
2001 Lexus GS300 (spousal unit's daily driver)