I have a 1999 triton tr18 with a 1999 mercury 150 efi. Recently I had trouble with the prop "so I thought". Several weeks ago I spun the hub in my 25 pitch 4 blade trophy plus prop. I purchased a new hub and ran it the next weekend. That day I fished a tournament and on the way back to the launch the engine started acting up again. I thought it was the prop hub. When we pulled the prop off there was metal chunks inside the lower unit. We pulled the lower unit off the engine and looked in the case and saw the exhaust tube busted. Well we ordered new parts and my mechanic pulled the powerhead and replaced the exhaust tube. A total of 500 dollars later. Well I have ran the boat 3 times and tonight when I was headed back to the launch the engine starts acting up again. I pull the prop off and see metal chunks in the lower unit. I am guessing that it is the same problem. I need to know why does the exhaust tube keep busting like this?