Misfire and cold engine RENAULT MEGANE I Classic (LA0/1_)

I bought a used Renault megane of an old man which had owned the car since it was new 1997. I bought it really cheap and i soon realised that something was wrong with the engine temparature since it only got hot when you let it idle for a long time, city driving etc. And also i had problems with misfiring, so i simply changed the sparkplugs and that issue disapperad, the heating problem did not get resolved. But now only a month later the same missfiring issue returned. When idle or on low rpms the car missfires badly. If i pump the gas it disappears and runs flawlessly. So what should i check first, i have already bought a new thermostat for the temparture, but what should i do about the missfireing?