If you let off the gas and it goes back down to idle your torque convertor isn't locking up...It also sounds like the od isn't kicking in like it should. Do you use the OD button? Keep out of OD unless you're going over 55mph. If you still have the same condition (letting off the gas and it goes to idle) you may need a converter or a transmission control unit... Have you checked for any codes? Check to make sure your transmission wiring harness isn't against your downpipes.. !!