my motor has gil dry pipes out the back with the 2 water dumps on the transom the 3 tiny holes on the inside toward the end and the 5/16 drain hole on the bottom at the end . I lifted the hatch at idle and it sounded a little like a coffee brewer. The manifolds were acceptable temp. the risers were hot, the temp. gauge was functional and showed cool as a cucumber. I'm wondering if the h20 is being exited too fast and was considering blocking the 5/16 drain holes as well as restricting water flow out of the dumps. Of course one step at a time. Also when flushing the motor it will suck the hose flat as if it can't get enough above idle. In my life to the best of my knowledge my girlfriend was able to do that, but that was years ago and I'm not 100% sure I could have even been dreaming.

Yes, I think you might be on the right track--- try using a little screw or silicone or something in the drain holes to dam up that cooling water a little and give it a try. Let us know what comes of it.