I am not a connoisseur of the Marelli system but indeed it seems that the ECU has a problem with the new ATS. 200 Ohm difference is not huge, but a difference nevertheless.
I guess the best would be to find the exact temp/resistance values and maybe put a fixed resistance in place to see what happens.
I am going to concede defeat on this one and run with the old ATS that works. I should have taken the “if it isn’t broken, don’t fix it” approach right from the start. Fortunately, i can return the new ATS for credit.
Thanks for your help with this mysterious behavior. Marelli and Lucas never fail to amaze me.