Ford Escort stumbling and high revs.

Hi all

I have an R-reg ford escort and have been experiencing a problem with the revs and some stumbling just after it starts. It Revs high then drops down then goes back up again this lasts for about 1 minute. Then engine seems fine. If I put my foot on the accelerator the drops in revs and then shoots up. This seems to clear after about a minute. When I pull off I have a lack of power and the car stumbles along unless I put my foot right down to the floor.

I have also experienced this problem whilst driving at speed on the motorway where the car seems fine and all of a sudden I have a loss of power and the engine stumbles along with the whole body of the car shuddering.

Does anyone have any idea what this could be?

Thanks

Paul