2000 Questions for all other cars Stalling

Engine Performance problem
2000 Questions for all other cars Two Wheel Drive Manual 76000 miles

I own a Rover 45.

When I start the engine from cold it Start/runs perfectly, after a minute or so it will start to choke/idle irratically, if left will stall.

If I set off straight away, at first it will be fine, then there will be no power. Once the engine has run/warmed up it works fine.

This problem also occurs when the engine has run for sometime and then left for about half an hour or so, but the power loss is quite severe.

I have had to computer specialist look at my car, one said there was 50+ errors and the could not be cleared, there other one said he could not fine anything wrong, even when the created an error.

I have also had the CPU tested, no errors report

Can you advise on what the problem might be.