car acting funny when i turn off ignition HELP!!!!

sometimes when i turn off the ignition on my car it starts shaking and making knocking noises and sounds like its still trying to run, and then it finally dies...what could cause this?

It would be helpful to post your year, model and engine size.

1969 buick skylark w/ 350

It sounds like carbon knock from carbon heating up allowing the engine to run after shutting off.
Try dripping some seafoam into the carburator when it is running. Available at Napa auto parts stores.
Make sure the motor is tuned up properly to prevent reoccurrance.