MY FOC trans spins smoothly and freely but it won't exactly keep spinning. What I would do is try to isolate where the problem is. Take the trans apart, take everything out and only reinstall the input and intermediate shafts and gears. Hold the case together by hand and give the input a spin. If there's a problem you know its with these gears. If not open it up, take out the input and try just the intermediate and the output.

Chances are the problem is with the intermediate and the output. When you installed the gold gear its possible the mesh with the output was altered. The gears for the output are bevel gears just like the diffs and can be shimmed the same way. You can alter the mesh with shims on the intermediate shaft, behind the pinion on the intermediate shaft or on the output shaft itself.