In the Aeolian, it is very susceptible to tempo slide rod alignment.
If the rods are either bent or not sliding in and out, very true -- the
tempo pallet can very gradually "lift" off the slide, creating a false
tempo.

You said the tempo gradient was faster than called for.  So I suspect
that is the case.  The tempo slot was designed linear for the tempo
indicator.  So the only way it would not be linear is when you add a
graduated leak.

Now -- if in case you are wrong -- then check the "reroll" lever
controls.  If that is not sitting solidly over the reroll hole, then
it will add to the tempo.  So if your air motor isn't the best, you
then won't notice anything until you reach a certain plateau.  After
that, everything accelerates.

Craig Brougher