[asid61]

12.6fps free speed for high gear is nowhere near the max speed for a 4 cim setup. We ran 15fps on 4 cims easily last year, and bumped it up to 17fps later with no problems (at least not on the gearbox side).
Are your c-c distances correct on the gears? The situations you describe are high-torque situations, and the ratcheting sound might be gears slipping. Get out some calipers and measure the c-c distance. Also, try running it without load and see if the issue persists.