Hi Philip
Sounds like it may be a failing ECU.
The exhaust would be wet and sooty.
An auto electrician found mine to be delivering 4 jets of petrol per rev instead of one, when engine cold but only at low revs ie <1000. When warm and revs above 1000 it was somewhat normal. Needed oscilloscope to diagnose this
Gradually deteriorated until car undriveable hot or cold
Replacement ECU was the answer.
Diagnostics did not show any fault. As the deterioration was gradual took month or two to reach this conclusion !
