The only time my 560SL gets a bit rough at idle, snorts a bit, or gets glitchy in the AC system is when it's not been exercising regularly.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)