2000 Pontiac Grand Prix MISFIRE AND JERKING PROBLEMS PLEASE

Hi I just got a 2000 grand prix se 3.1 with 179k and the car seems to have a misfire in park or at low rpms (every once in a while it will happen when I am driving). When the car warms up after like 15-20 mins it will seem like the trans is jerking in the first couple gears sometimes even third. When the engine is cold it will run just fine. Also the car seems to be eating gas like it is nothing. Since I have had the car I have changed the oil and oil filter, Motor Mounts, Trans Mounts and also the plugs and wires. Im not sure if this matters or not but when I changed the plugs I used Motor craft. My friend who works at Pep Boys said they were the best and that it didn't matter. So I decided a couple of days ago to pull out the new motor craft plugs. I noticed that they had not been gaped right and also that they all had oil on them. So I gaped them to.60 as specs say to do. The car ran great for about 24hrs the trans shifted perfect no misfire or nothing then it all came back like it never went away. What could this problem be? The car has no check engine or trans light on I tried to see if any codes would come up on the OBD sensor (but none did). I am so lost right now anything could help me at this point. Thank you very much Scott