I had the same problem with the throttle
If your trani shifts fine at the engine, then it may be either the cable, or the connection. My throttle handle became very hard to move. If I work it hard a few times, it will get a tiny bit easier, but not by much.I tried using lube in the cable and that didn't fix it. By the way, always use graphite for the cable. Don't use oil of any kind, even WD40, because they will collect dirt and get gummy inside, and you will then HAVE to replace the cable. Graphite comes in a spray form and drys very quickly. You can probably use SailKote too.I finally tracked the cause to the connection where the cable linkage meets the engine. That connection is suppose to be rotate freely, but because of a slightly loose hose clamp just above it, some salt water dripped on that connection, and some rust has started to form. So I tighten the hose clamp, took the connection apart, sanded and scrubbed with a old toothbrush, put it back together with some lube oil. Now it has been 3 years and it has been working great ever since.