Chevrolet Repair: 1995 Chevy 1500 Z71, 350, 4 speed automatic, 1995 chevy 1500, oxygen sensor

Question
My truck feels like its trying to shift gears when i am cruising between 40-50 mph.  The transmission will be in drive, and it occurs more when the engine is really warmed up (30 or more minute driving).  The RPM's will drop from about 1500 to 1000 rpm and immediatley go back up and it feels like a shift.  It will do this rapidly if you are holding the accelerator steady.  Other factors:  The transmission is rebuilt with 4k miles, it does not do this when out of over drive, it has a new TPS, the EGR system never has worked right.  It is disconnected right now.  I have tried to replace the valve and the vacume solenoid with no success.  Since it is disconneted the check engine light comes on after 10-15 minutes of driving.  Could all of this be the ECM or something total different for the "transmission shift" effect as i call it?

Answer
Sounds like you have a lean condition.  Check your throttle body base gasket.  Also an oxygen sensor that has a false reading could cause this.  Your intake gaskets could be leaking internally as well.  As far as the egr goes the passages in the intake are probably plugged allowing no flow.  Carbon can also affect vacuum passages in the throttle body.  Still no success see a dealer