2009 p0304 error code rough idle
 

Car once hot idles horribly. Can turn it off and back on and will be ok. Finally threw a code. Replaced water pump and thermostat a year ago at 75k and now have 95k. Could it be as simple as clogged or carboned valves or is this much more serious. When car is only worth about 5-6k, expensive repairs really aren't an option. Thanks for any advice.