I have a 1991 b2600 4x4 5spd ext cab with 106k mi. I replaced theECM with one from NAPA about three years and 30k mi ago.Until recently this truck hasbeen used mostly for driving around town to class and work, not any long distances. However since the beginning of the summer I have been driving it back and forth from an internship every weekend which is about 300 mi round trip. The truck runs fine and gets pretty good milage (low 20's) on these long trips but I have noticed a problem recently. When the truck has been sitting overnight it starts up fine but idles rough and smokes some for a few seconds, like it is not hitting all cylinders. It goes away quickly, but I want to take care of this problem before it gets worse and the truck is not drivable. After the inital start of the day this problem does not exhibit itself again. I replaced the spark plugs (which were worn out) and put a bottle of fuel system cleaner in it which seemed to help for a few days, but the problem is back. I was planning on next replacing the fuel filter and the cap, rotor and plug wires. I have a bad feeling that this could be the beginning of ECM troubles again, which I know is easier to have fixed before it gets really bad. I was wondering if anyone had any advice on this or knew how to test the ECM before I spend big money getting it fixed/replaced. Thanks in advance.