Rough Idling 560SEL
 

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.

You can have an indy shop does a vacuum test to see exactly where the leaks are. They inject smoke in the system and if there are leaks, smoke will come out. The cost to the the test done is about $80.