Outboard issues

[Kent]

I have a 1992 150 HP Mercury Black Max (oil injected) outboard on my boat. It has been running fine until the last two trips. It starts fine and RPMs are not an issue while it is out of gear. When I put it in gear it will not gain adequate RPMs (only goes up to about 3,000 and should reach around 4,500 to 5,000). If I quickly shift it back to neutral, while it is in gear, it will die. I pulled the plugs and they looked fine, but I will replace them anyway. I am also replacing the fuel filter. Are these likely my problem or should I be considering something else?
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