The timing chain and guides may need to be replaced, but it's hard to see how that would affect the idle.

What it sounds like is happening is that for some reason, the idle is dropping and the relay is telling the air valve to pump more air to bring it up. Then it gets too high, and the relay calls for less air, the idle falls, and so on.

So there can be something causing the idle to fall, like a vacuum leak or a bad injector. Or the air valve or the relay can be bad.

There are very straightforward tests for fuel pump delivery, injection pressures, vacuum leaks, and idle relay/air valve function. You should ask if these were done, and what the results were.

__________________
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