I have a 99 Tahoe with a 5.7L and last week the throttle stuck. So I replaced the throttle cable from the pedal to the throttle body. While I was under the hood I changed plugs and wires as well. Note I replaced the wires as I took them off to avoid crossing. After putting everything back together it ran like a champ. I test drove it for about 5 miles around town and it ran so much better then before. My girlfriend then drove it 12 miles back to our house and once it got there it doesn't want to idle. It idles like it would if you disconnected the MAS. however everything was connected and fine. I took the MAS apart and it appears ok. Any suggestions on what it could be? I don't want to spend the 105$ on the new sensor unless im sure that's what it is. I hooked up the reader and it read 1 code being MAS issues and the other 2 were showing cylinder misfires.

 

Have you checked for vacuum leaks?