Triangle...Try this link for a Perkins compatible oil cooler...they are great folks.
Eleanna...sounds like same symptoms I get when I pick up a crab trap! Suggest you check your cutlass bearing for binding/wear as this could be responsible for the vibration and engine problems.
I assume you HAVE checked that you have good water flow in out of your exhaust AND that this water flow increases as you increase the throttle RPM's. If the flow does not increase then you could have the problem DynaMeme describes OR you could have a restricted flow somewhere in your water INTAKE line/filter/through hull. I would check this before hauling the boat to check cutlass bearing and prop.