2003 Isuzu Axiom vehicle shutters at times when acceleratin

Prior to the shuttering we noticed that everyso often when letting off the gas on level or downhill roads the transmission would down shift then if you tapped the gas it wold go back to the previous gear. Now the vehicle seems to shutter only when attempting to acelerate from a stop. High and low speeds there is no noise, smell or sound comming from the drivetrain. Trans and transfer case Fluids are new as well. When at a stop posittion sometimes the vehicle seems to idle up and when comming to a stop it seems to idle way down almost to a stall. One dealer says its an internal transmission prob. however another 2 believe it may be a PCM/ECM. The car just passed emissions however my wife tells me that pep boys recomended replacing an O2 sensor about a year ago dont know if that could be part of the issue. No check engine lights. But I am going to get advance auto to run the scan for me to check for any codes any way. Any suggestion on the possible cause of this or where to start prior to dropping the transmission an a few thousand dollars.