Hi Fred,
I concur with Marl's suggestion about changing the filter as a first "to
do".  OR, at a minimum, dropping the Trans. Pan, inspecting the Screen for
sludge and/or other particles. Depending on what you find on the screen will
dictate what to do next..... A pure speculation on my part here is: The
synthetic fluid may have "dissolved or otherwise put into solution" a bunch
of sludge from the internals of your transmission. In addition, the so
called dead volume of the Auto Trans...... the fluid one cannot remove by
way of gravity..... that resides in the Trans Cooler, Torque Converter,
Clutch Packs, etc is a significant volume. All this means is that some of
your old Trans Fluid is still in there, diluted with the now 1k mile old/new
Synthetic Fluid.

If you find a moderate to allot of sludge and/or particles on the screen,
the valve body will need to be removed from the Trans. and cleaned. Not a
hard job but,  an intense job (Covered in the Bentley). A best case
scenario: clean the Valve Body, replace the drained fluid with your choice
of Synthetic or Dextrin II and hopefully symptoms will disappear!
Good luck Phil Z.