My 1993 124 Series 320E is experiencing intermittent problems where it refuses to switch from 4th to 5th gear. Win Star engine diagnostics reveal fault code 06 Can, which initially suggested faulty ECU, Wiring Harness and Data Bus link. All these items have been replaced and the ohms across the various circuits tested as ok but the fault still persists and seems to trigger when the engine revs above 3000rpm. A test loom has been connected to the ECU which seems to alleviate the problem at least for a while but the fault eventually returns. A donor vehicle is available and we have tried all the other ususal usual suspects such as changing the gearbox control unit and the idle speed actuator, all to no avail.
Any ideas or advice would be greatly appreciated.

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1993 320E 5 Speed Auto Sportline