As Doug Johnson mentioned it may be control stiffness encountered at
altitude resulting from either the effects of pressurization on the controls
or colder temperatures aloft.

I once flew a Seneca III out of Rochester NY on a COLD winter night and
after I climbed to about 9000 msl the autopilot kicked off and I could
barely move the elevator (stabilator actually) trim. The problem turned out
to be that I had lubricated the trim jackscrew with the wrong kind of grease
!

After cleaning the trim jackscrew and drum and relubricating it with the
proper low temp grease the problem never happened again.

I agree with Charlie Kohler, you may have servos with slipping clutches but
I can't say I'd recommend just increasing the servo torque to overcome
another more basic problem.

Ted Stanley - A&P-IA