how often doesa 1990 300 need a tune up do you reckon?Ibought one a year ago and it ran fine for 7 months and then started the typical idle stuff...I have replaced the fuel distributor, coolant temp sensor, assorted vacumn lines as well as cleaned the idle control valve in addition to re-gapping the plugs (which were way open). I was told that the thing was just tuned up before I bought it but I now am questioning this (same dude tried to pawn it off as a 4Matic until I looked under the front end, he plead ignorance and said he was told it was down in florida, I plead poverty and told him $2600 instead of $5200, he took it (my 1st MB so I didn't realize it should indicate 4Matic on the trunk)). The thing has a minor tremor/shudder at idle, exactly like it needs a tune upbut I've been dancing around it because the seller SAID it just had one.Car still runsOK but it's not right, so far spent less than $100 for the above but I have this nagging feeling it needs a good old fashioned Bosch tune-up...car has 125K onit, burns no oil and seems fine other than thisunsettling idle presence...any ideas? I know I sound like a dick for asking but the tune up parts would be around $250 plus I think it may be the spark plug wires anyway. As I am a cheapo I am always looking for the silver bullet....thanks for any help....cliff