Keep the beast throttled down for now.
While I have owned an A4 engine for 10 years I'm not so familiar with the gear box/clutch so your next stop should be here: Moyer Marine Atomic 4 Community - Home of the Afourians - Powered by vBulletin
and you can ask that group about how to adjust the gear box/reversing gear assembly.
That group knows the Atomic 4 engine, inside and out.
Have you checked the shifter cable to see that it is not slipping somehow? Could be that simple but somehow I doubt it.