How is it running (e.g. very rough?, misfiring?, or (smooth...but just not following the accelerator) when it slowly climbs back up to 2000rpm with the accelerator on the floor and drops back to 800 rpm? If you slowly move the throttle as it's doing this, how does the engine speed change? Does the rpm ever start tracking the throttle movement while it's trying to recover?

Does this truck have the vehicle stability control (and the drive by wire throttle control)? If so, do you get any lights indicating the VSC is active? (i.e. erroneously thinking it's in a skid, and taking control of the throttle?

Puzzled that no codes are being logged into the computer.. that might be a clue that the VSC has gone crazy and taken over control of the throttle.