When this deck changes play direction, the capstan belt shifts over to a different position on the flywheel. There is a "step" on the flywheel where the belt rides on. This step slows down the tape-entrance capstan, causing a speed differential between capstans and thus tension across the heads. When you change the capstan motor direction, the belt shifts over and the opposite flywheel then slows down. Make sure your belt shifts every time you switch direction.
Why did you buy parts from Japan? Teac of America has never stopped selling the original belts or pinch rollers for these decks. Once again, make sure both of those items are the exact Teac part....it's critical for proper operation.
Make sure the tape-tension changes when you reverse direction. The take-up tension should be about 10 grams more than supply and this should switch when changing direction of play. Specifically: 50 grams take-up, 40 grams back-tension.
Also, when you installed a new capstan motor, did you reinstall the two washers under one of the motor-mount screws? It tilts the motor, causing the belt-shift I described above. Without the belt-shift, this deck will not operate properly.
I've probably encountered over a hundred X-series decks in my shop with this defect (it's very common) and all have been repaired by addressing the above.