My last Mitzi had a similar problem, it ran perfectly cold or hot, but warm it would stall all the time when you took your foot off the accelerator. Frightening when approaching a road island or changing down for overtaking. You got suddenly less effective brakes, no power steering assistance. It was fuel injected with a manual gearbox.

I gave up and took it to an auto electrician. He fixed it, not cheaply. Claimed to have replaced something called the Stepper Motor, an expensive item that acts as an automatic choke on F.I. motors. It has three settings, cold-warm-hot, and was jumping the middle setting.
Hope this helps.