[DiGGM95]

Maybe EGR, plug wires arcing, or even fuel psi. To check the EGR (its on the back of the intake manifold, kinda off center toward the drivers side, like a lil UFO). Remove the vacuum line from it, depress the diaphram and put your finger over the vacuum port and release the diaphram. If the diaphram dont hold open until you remove your finger, the valve is bad. Also open the hood with it running after dark to check for obvious plug wire arcing. Btw, have you had it scanned for codes? An EGR problem commonly causes a code. Get a fuel psi gauge too and see if it produces over 42psi at idle.