Juddering, Over Revving, Cutting Out - When Wet?

[Fifi_2001]

I have a problem with my 2001 Ford Fiesta. When it rains or I take it to the car wash it doesn't run properly. It over revs when I put the clutch down and then when I come to a complete stop the revs go down so low that it cuts out. I've noticed that when this does happen the car consumes more petrol, I find I can lose around 100 miles off a tank! It also judders while I'm driving along which feels like the power has been cut and then it suddenly bounces back which causes the judder/jerk feeling. I've changed the coil pack and leads and the spark plugs have been checked and they are fine. I took it to the garage and all they could think it could be was the throttle sensor (he did though say that he couldn't understand how wetness would affect it), this was changed, the car has since got wet and its back to its over revvinng, cutting out, juddering self again!

Does anyone have any ideas as to what is wrong with my car?

Thanks lots

Kelly