I have a 3.8 liter v-6 engine. When I start it it rums fine. after it warms up it acts like it is missin. I did a complete tune up on it and still no change. I can not advence the timing anymore than what it already is and if I turn it down the car stalls everytime i stop. The car only has 65000 miles on it. It is a 91 thunderbird. I checked the vacuum lines and had a ecu readout done. It showed no problems.