I don't understand what your mechanic means when he suggests it's a computer "compatability" issue. When one of the computers was changed out a year ago, was it not replaced with an identical model computer?
If the car starts up fine and idles nicely when it is cold, but when the engine is warm the starter motor turns but the car doesn't "catch" and idle, it sounds like a relatively simple "warm start" problem.
I agree with sloboy89
's comment above, in that your mechanic needs to follow the L-Jetronic Diagnostic Guide
that was put together by Greg Gordon and Eric Russell and is located here: https://www.hpsimotorsports.com/l-jetronic--spider-
Tell your mechanic to pay particular attention to Step 3 the cold start and warm up system
because a warm start issue may be due to something as simple as a faulty TTS
(which would make the CSI spray and flood the engine when warm) or an AAV that was stuck in the open position.
A more comprehensive study of the theory of operation, troubleshooting and service of your car's injection system is presented in Bosch Fuel Injection & Engine Management
by Charles O. Probst ISBN 0-8376-0300-5. Your mechanic should have a copy of this book (or something better) or he may not be qualified.