My car keeps shutting off when it idles. What couldbe the problem? on 1994 Ford Escort

It started after I filled the gas tank on saturday 12-24-11. The car was working fine I drove it to work with no problem now whenever I start the car its fine then once it is sifted in to drive or reverse it starts shaking and shuts off. This also happens when sitting at a light.

Asked by for the 1994 Ford Escort