1988 Chrysler Le Baron Car kills at idle

Hi. Well my mechanic said this was the only car in 20 years he could not repair. I'm hoping it's something so simple that he is just overlooking the problem. The car started rough idle at red lights or in traffic and progressively got worst. It now will not stay started, unless you keep the engine revved at about 2000 rpm.
Something strange he found while working on the car was that if you disconnect the alternator from the battery the car will run smooth at idle. As soon as you plug in the alternator the car will kill. Alternator was changed and did not fix the problem. With the car running at 2000rpm and the alternator plugged in, I checked the voltage on the battery and it is 13.7 volts. It seems the charging circuit is working properly, right? With the car running on just the battery the voltage is 12.3 volts. Why would plugging in the alternator kill the car? He also changed the computer just as a last resort and it still did not correct the problem. Thanks in advance.