If you don't have the service history, I would recommend replacing the timing chain, upper slide rails, plastic oil tube pieces, and chain tensioner.

If you do this, and don't see any obvious wear on the cam lobes or rockers, then you probably have balls studs that are leaking down and not holding the rockers tight against the cam lobes. I would try "part synthetic" oil and a couple of 500 mile oil changes.

The miss at idle could be anything from cap/rotor to a plug wire to an injector to the fuel distributor to a vacuum leak. You need to work your way up the "expense chain." Lots of posts on doing this.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe