Whats up with this Vectra?

We have a 1999 mk1 Vectra with the X20DTH engine, over 225,000 miles on the clock!

Just these last few weeks for some reason, the throttle cuts out altogether. It only seems to happen when the engine is warm, if you lift the foot off the accelerator pedal. It doesn't cut back in unless you pump the throttle pedal.

As you can imagine this is pretty dangerous if it happens at a junction. There is no engine management light on when these symptoms occur. The problem seems to be getting a lot worse, at first we did think that it was down to wet weather, however these past few times driving it, it has happened when the weather is dry too.

Any ideas as to what this could be and if possible roughly how much are we talking for it to be sorted?

Many thanks.