ok this is the deal 2000 silverado 2500 4x4 6.0 gas
idle drops when the trans gets 2 temp 500rpm to 300rpms only in gear. now what iv done iac, rubber grommet for the butter fly, clean TB took off truck cleaned very good, egr, plugs, no dummy light

 

One thing I've learned is that when you clean throttle body, disconnect the battery, remove the TB to clean completly, reassemble, then re-connect battery. When the TB gets gunked up, the PCM changes a lot of settings, so when the TB is cleaned and gets cleaner air, the PCM has to be re-set (why to disconnect battery) to re-learn all atmospheric settings, again.

hope this helps.

 

did a reset still not fixed