Engine misfire... what could be wrong with my 2004 ford escape?

so i noticed when i was driving my engine started to misfire... i hadn't driven my car in a while and it is really cold here where i live. when i was picking my son up from school i was stopped but not in park... that's when it started chugging as i call it. the next day i had a friend come check it out and he said it drove smooth. it was not going into gear when i drove it. now it's driving fine, but when i have it in park and hit the gas the engine starts to do the "chugging" misfire thing again. I have no clue what could be wrong. i am planning on getting a diagnostics test ran, just wandering what i could be looking at here. my transmission fluid is fine, mind you! what an awful time to have car problems!