I had a similar situation with my 200tdi, no immobilizer but same symptoms. Mine was a bad stop solenoid wire connector. Replaced that and no issues since. Just throwing another idea out there.

Oh and you are describing my exact issue which is why I advanced the timing when I did the timing belt change. Fixed engine and EGT temps straight away.

Advancing the timing is easy to do, so easy even I did it. Yes remove the access plate, making sure the engine is at TDC and that the pump is lined up. I used a 9mm drill bit. There is a nice video on you tube about it. Just google it and watch the one from trailfitters toolbox (no affiliation)...