While idling, most of the time my Blazer will randomly act like its about to die, but then smoothes out again. It'll do this 3 or 4 times and then run sorta smooth for a while. Replaced fuel filter and pump, air filter, oil, spark plugs and cap+rotor. Only thing I haven't changed yet is the plug wires. I also took it on the highway for about 12 miles (my first real drive with it) and I noticed that it shakes a lot around 40-60 MPH. you can feel it inside the truck and in the steering wheel. I have no idea what would cause both of my issues......