2001 Dodge Durango LOW RPM

Engine Performance problem
2001 Dodge Durango V8 Four Wheel Drive Automatic

start it, idles fine drive one block stop at the corner rpm drop down 400-500 stays there until I tap the gas bring the rpm up to 800-900 it will idle off at 750. I have checked all vacuum lines also throttle body spacer for leaks, cleaned throttle body and positioning sensor. Did a tune up only changed plugs and air filter maybe there is more I should have changed?
please help really stummped on this one
if it helps had engine light come on friend got code first gear ratio thought I should change tranny filters to see if any improvement. Have them now still not installed. While I was waiting took it for 2 min drive down the high way after cleaning off tps and the check engine light went off still same low RPM problem. Also light came on accelerating in first gear didn't want to go into second just reaved like a huge tranny flair thank you for ur time :-)