Have to remember some isn't as larned as us.
"as I accelerate to get to the next gear (after the shifting was completed) it slows down," that sounds exactly like a failing TPS. Yoy press on the pedal but the ECU thinks you took you foot off. Dead spot maybe ? Why you need a scanner (e.g. Torque Pro) that can monitor and keep a log. Intermittents are hard to find. If steady will set a P0122 or P0123 but intermittants are too short to set a code (and "foot off" is a valid reading).