This is on a 90SPL J90TSLECM. Motor idles great then a noticeable tone shift, rough idle and tach drops. If you rev it the tach signal comes back; sometimes it stays idling rough with no tach reading and sometimes it returns to a good idle with tach reading. However, eventually the idle always returns to rough. Not sure if it matters but it sheared the flywheel key a while back and the aforementioned started on the first time back in the water after installing a new key. Ay thoughts on what this could be or some good trouble shooting tips?