You can run it slow forever and it will never overheat even if there is absolutely no raw water coming through. When its under load engine makes heat, and even the smallest issue in the cooling system (usually raw water side) will show up as high or raised temp. Other guy was saying to check raw water pump. I would suggest you check raw water flow at your exhaust elbow (exit point), see if there seems to be enough water coming through. I had issues with clogged aftercooler element, and engine would shoot to 200F in 2 minutes under load. I could cruise all day under 2500rpm and temp would stay in 175F range