Only in hot weather? The first thing I'd check would be cable tensions, although they'd have to be very high to cause that amount of stiffness. Do you hear any creaking from the tailplane on your pre/post flight checks that might indicate interference or damaged stabilator bearings?
Sometimes notchiness can be caused by the control handwheel tube being dirty and dragging slightly in the support bearings. That wouldn't affect the stiff trim, though.
A trip to the hangar, I reckon, RF.