Hard Shifting Merc Optimax 150

[TripleCrownNC]

I have twin 150 optimax motors, 99 models. The throttles have always been a little stiff. Boat had little use for 5 months, now very difficult to shift starboard engine. Removed shifter cable, thottles loose, install new cable. Thottles loose with new cable pulled, until I attach to motor. Now very difficult again. Seems the shift mechanism at the motor causing the problem. What next? Thanks, Captain Dave.