1991 Subaru Justy Engine Idling

Wow, what a great site! I have a Subaru Justy 3cyl 1.2L fuel injected 2wd. I recently purchased it with only 23,000miles! The only problem that i'm experiencing with this car is that it starts right up and runs and idles well when it's cold. But after drving awhile, the engine idles roughly, the rpm shakes up and down, and a loud knocking noise comes from the engine. This only happens at a stop. Otherwise, the car runs very well and no problem. Can you suggest what it could be? There's no oil or any fluide leak and the engine is very clean. The only thing that i've done was change oil, replace thermostat (heater wasnt' working along with temp gauge). Thank you so much!