Northern Lights Gen overtemp under load

[La Vie Dansante]

I have a Northern Lights Diesel Genset, model M773LW3 on my sailing catamaran. It holds steady at the normal 185 degree temp when just charging the batteries, but recently started overheating after about an hour or so when additional significant load is applied (air conditioner, water heater, etc.). Previously, it could run these loads without overheating.
The generator itself is located in a forward locker, just inside the starboard hull, while the raw water strainer and through-hull are located in the port hull. The raw water strainer assembly includes a MS1200-6B circulation pump that I'm assuming is there to assist the generator's water pump">raw water pump to overcome the distance it must move water through pipes from the port side to the starboard locker (about 16 feet). In addition to the overheat situation, I'm now hearing an intermittent (every 10 seconds or so) hum from the circulation pump when the generator is running. Could this pump be failing, causing the water flow to slow, preventing adequate cooling for higher loads on the genset?