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When this occurs does it seem like the engine has the hiccups?
Does it feel like a 1-3 second fast vibration?
Does it seem worse in OD when going up medium grade hills around 45 mph kind of bogging it down enough to need to downshift?
I believe your describing a possible torque converter clutch shudder or a subtle misfire from a weak ignition coil. i suggest you have a power balance test ran while driving to see if it can pick up a cylinder thats providing less than the others as well as use the scan tool to match up the turbine shaft speed and RPMS to see if there is more than a 3mph difference.