Hi fellow boaters, i've got a 21' daycruiser with an Olds 455, TH400 tranny and a casale v-drive. I bought the boat without a test drive (ya I know not the smartest thing to do). For some reason the tranny shifts automatically, just like a tranny in a car. That is, when it comes up on plane or when I slow down for a 5 mph zone, the tranny shifts down from third to second. I'd like to adjust the tranny so it stays in the gear that the shifter is in. Kind of hard to use it as a ski boat if it is shifting unexpectedly and I can't keep the speed constant for the skiier. I talked to a tranny shop and the guy suggested I adjust the vaccuum modulator but I don't know if that is the right thing to do - he didn't seem to know much about boats so I am reluctant to rely on his advice. Thanks in advance.