engine wont idle below 3000

Hi

On the weekend I was driving my car "sitting in traffic for only 5mins" when my car just cut out.

I tried to restart it but it would not, finally it took but only idled below 500, while idling it was very rough and felt like the car had a heavy miss"hole car would shudder". I took the rpm up to 3000 and then the engine ran fine but once it dropped below 3000-2000 it just died.

So far I have by passed the fuel filter, replaced the spark plugs and the distributer cap...

Any other ideas what this could be, dont want to send the car in and have to spend to much repairing it.

Its a opel kadette 1992 model

Thanks