OK, thanks. I know it's a chevy block, but who actually made the engine? I assume since it's 1989 it is carbureted. That difference in RPM suggests that you have a fuel system problem (you didn't report any missing, backfires, etc. so I assume ignition is OK). The first thing to check is your fuel filter. If it's the canister type, empty it into a jar and look carefully to see if there is water at the bottom. If so, that is probably your cause. Put some water absorber (EZorb, etc) in your tank and try to get all the fuel used up. If there is no water, make sure the filter is clean - replace it if there is any doubt. The other possiblities are that the fuel pump can't keep up with demand and may be failing, or the carb needs to be rebuilt.
Marine Mechanic
USCG Licensed Captain, 20+ Years Boating, Fishing, Maintenance and Repair Experience