2000 Evinrude 5hp 4-stroke Cooling Problem

Water streams out fine except when at or near lowest speed for trolling. Spits and eventually stops streaming at low speed then smoke or possibly steam seem to come out instead of water. Eventually the engine dies. Starts again easily and throttling up, the water streams out again. Runs great a high speed, but throttle down again to troll, and then the water stream slows to a spit and eventually dies again.

I don't know how the cooling system works but I'm guessing that the impeller in the water pump that might be worn. It's never been changed and from searching on the web, it looks like that is something that should be done every couple years.

Where can I buy the parts I need and how difficult is this to replace?

Thanks in advance for your help!