3.1 high idle, runs rough

My Buick Regal 3.1 recently quit on me. It started right back up, but... it now runs rough sometimes. Going down the road it will act as though it is on cruise. You bump it into neutral and it is at high rpms. as soon as you stop, it goes to a rough idle. The IAC, TPC both have been changed. New plugs and wires. Also, maybe unrelated, it ran out of gas on me, but showed a little less than half a tank. It has done this several times. I put fuel in tank and it starts back up. Car hold 16.5 gallons and on fillup, it only held 9.3. With what I put in it out of can (2.5), it should have had at least 4 gallons in tank! Go figure. I feel it has a vaccum leak somewhere, as my brakes get a little hard when trying to stop with high idle going on. I realize that there is less vaccum at high idle. You can unplug hose from brake booster when idling rough and car actually runs better. Put your finger over end of hose and it dies. I have had advice from fuel pump, regulator (fuel), cracked intake manifold. One more thing, when car is cruising at, say, 55, it will drop in and out of overdrive. You can press brake when car is in overdrive and the torque converter will unlock. Just need advice. Pressure from pump has not been checked yet. Most of time, when car is started, it will immediately go to a high idle. This sounds like a vaccum leak. PLEASE HELP!

Check your transmission vacuum modulator. It is located on the transmission toward the front of the car. It has a vacuum line going to it from your intake. Remove it from the transmission. If any tranny fluid comes out of the vacuum hose fitting side then it is bad. When these go bad it usually feels like coming to a stop with a manual tranny without disengaging he clutch. When this part went bad on my '95 Regal GS I didn't experience this though (probably because my tranny - the 4T60E - is electronic). My tranny just shifted a smoother after I replaced the vacuum modulator. The torque converter is actually supposed to unlock when you hit the brakes. I would also check your MAP Sensor and your Purge Valve/Solenoid.

Ditto.