Sounds like a vacuum leak problem.  It sounds like the blower is
slowing down because the various vacuum controlled doors are starting
to move to their un powered positions when you go uphill and the
vacuum from the engine drops. That makes the air start to go out of
places under the dash instead of the dash vents and that makes it seem
like the blower is slowing down.  It could be you have a bad check
valve, they are usually under the hood in the main vacuum line to the
AC system, you could have a cracked vacuum canister/reservoir, or you
could have a vacuum leak in a vacuum line under the dash or in one of
the vacuum motors.  Those are my guesses.