200 TDI running issue

[spudgun GB]

Hi all,
My mate has series 3 lightweight that has been converted to 200tdi at some point in its life. We have a bit of a running issue with it that I have no idea how to move forward on.
If you leave it to idle for five minutes and press the throttle it won't pick up. It bogs down but as soon as you let the throttle off it idles perfectly. With a bit of feathering it will pick up again but if it was a petrol car it would be best described as what feels like fuel starvation. It also runs out puff at the top end and if revved nearer the upper part of the rev band it starts to lose power and it takes a second or so once in the next gear for it to come back to you. Right that's the symptoms.
We have checked that its boosting properly and holds 12psi well once up and running. I checked the fuel flow yesterday to see if we had an air leak or a dodgy lift pump. The flow out of the return pipe at idle was around 1 Litre/minute which I thought was plenty. After that I'm stumped.
Any ideas would be gratefully received.
Thanks

[spudgun GB]

Thanks very much for that. We will give all that a go. I read a post before on landyzone mentioning under the diaphragm but it wasnt very clear. Your is much better. I will let you know how we get on.

[spudgun GB]

Today we got around to having a go out what you suggested and I think we have cracked it. We gave the fuelling cone a bit of a turn towards the richer side while we in there. The old girl flies now. Thanks very much for your input.