What makes the mod appealing is while my guess cost you nothing, a shop will require a few hours to diagnose the problem then more hours to fix the problem. These engines have some built in diagnostics but they're a far cry from modern OBD. Having fixed that problem, the car continues to age and something else confuses the ECU. I'm sorry if this is a non-answer but it's likely to be the only answer you'll get unless you catch the attention of Steve, Donnie or one of the bonafide pros.

Sixto
87 300D