I have a Ford 01 e150 v6. The idle goes bump bump bump, like it is going to stall. It only idles like this when in Drive or Reverse gear. If I put in in Park or N, it idles normally. ALSO it only idles bad when it is warmed up. I changed the MAF sensor, but it continues to idle bump bump bump, when in gear R or D and warmed up.
It throws the Ford trouble code 0171 and 0174 SOMETIMES.

 

Rent a good scan tool

check LT fuel Trims Would be out of whack & ST fuel trims would fluctuate
Dirty injectors
fuel pump weak check fuel pressure
fuel filter dirty
EGR needs cleaned
check for vacuum leaks with some starter fluid
Clean your IAC valve

start here first