There's lots of stuff besides the computer.

In addition to a flakely OVP relay, you could have a bad idle air valve, or air leaks in the plumbing.

There are also a variety of sensors that provide information to the computer, which the computer uses to determine how much air the idle air valve should supply. If these are not functioning correctly, the computer will think that it should cut the power to the idle air valve. If this occurs at idle (instead of deceleration), the engine will stall.

There is a very large number of posts on this topic.

Sounds like your mechanic checked the basics. The next step is to drill down on the sensors. If it will actually die every time at idle, it should be fairly simple to find the problem.

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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