I have alpha 1 ( It over reved)

I would have the prop inspected first. Stock Mercruiser props have a rubber isolator between the hub and the body of the prop. When the isolator goes bad, from normal ware or even a mild prop strike, the hub can spin inside the prop causing the surge in RPM. Backing off on the throttle may have allowed it to grab again. This problem will show itself most durring heavy load conditions like full throttle starts. On rare occasion it has been known to happen even on new or rewoked props.

If it truly slipped "out of gear" you should have heard some grinding like somebody driving a stick shift for the first time.

If that is the case check for a streched or out of adjust shift cable.

Hope this helps.

Any other sugestions out there?

Boater2 be is right. It sounds like you need a hub kit for that prop. Boat US and West Marine carry them. When I had my Alpha 1, they were costing about 40 bucks a kit.

Sounds to me like you had prop cavitation, airbubble form around the prop or gets sucked into the prop and the prop no longer "grips" the water causing your engine and rpm to rev real high. Pulling back on the throttle slows the prop and allows it to bite back into the water. This wont cause any damage as long as you dont let the engine rev out of control for a long period. Nothing to do to fix this, happens on every boat.