It won't hurt you to have a spare onboard, even if it is not the cause. You can buy it from Volvo for around $400+, or you can find it online for $150+. Just to be clear, I am not talking about a flush of the coolant. I'm talking using radiator flush and following the directions on the permatex bottle. This will clean out all the tubing and block runs on the fresh water side.
What is the WOT on your boat with the prop in gear and the boat moving, not tied to the dock?
Paul L