I have an '87 K5 with a 350 TBI. Everything is stock. I had a rebuilt engine installed 11/99 and although it doesn't burn oil anymore, I still have a problem with the way it runs. It is very cold natured and has an intermittent rough, unstable idle symptom. After it warms up, it runs better but still doesn't seem to run as strong as it should. After I have been on the highway, it runs very smooth for the first 1 or 2 stops. When I get into traffic it starts to idle rough again. I have replaced the fuel pump and fuel gauge sending unit, throttle position sensor, distributor and all related ignition components. I suspect some component of the emissions control system but I'm not sure which one. The ECM has no stored trouble codes. Has anyone ever had a similar problem? If so, what did you do? Thanks.