car is running really rough

[n00b on demand]

Ok here is the situation....

Its about 35 degrees here...very cold morning. The coldest since the winter. When I start my car on cold mornings such as this one it will start and then the tach drops like the car might die but then it evens out and idles properly. Today thats what happened to me ...but the car stalled. So i started it up again and i let it warm up fully like i always do. Everything was going fine. I go to the bank to get money out...and then i start the car up as im about to leave. It starts running really rough...sounds bad too. When you give it gas and the tach is rising i dont feel any power loss and the car drives fin with no problem...but when the tach comes back down to 1000 rpms or under it starts feeling and running rough. I thought it was gonna stall at each light or stop sign but it never did. I parked it in the garage and i shut it down and started it back up again. It didn't go away...still rough. I had a can of fuel system treatment in the garage and put it in.

There is no Check engine light...and the car was running perfect before this happened today. I'm gonna buy some dry gas and put that in and disconnect the battery to reset the ECU. Be patient with me....im not an expert on working on cars so if i have to do anything I need dummy instructions. Thanks in advance.