[700rpm]

Possibly a bent shaft on your starter. Or possibly boogered up teeth on the ring gear. Or possibly a weak spring on your Bendix drive. I would not expect a modern Bendix to solve this problem. When the engine was rebuilt, was the ring gear replaced or at least rotated 90 degrees? Engines want to stop in the same position all the time, son the ring gear gets used a lot in one or two places.

You shouldn't have to R&R the starter when it sticks, though. Just put it in high gear and rock it back and forth; it should pop loose.