I have a 2001 Mercury Optimax 225 3L DFI engine. I don't generally go on the lake until the water is 45 degrees or above but this year I decided to get out there early. Here's my issue. The water is 38 degrees. When I launch the boat, I start the engine and go park my truck. When I get back to the boat it has warmed up to 118. When I drive to my first spot it runs between 112-118 which is fine. When I start it up to move it is around 100 and it warms up to 112-118 again. After 3 or 4 runs, when I start it up it starts to drop from about 100 to 80 or less and it won't go back up. It has been as low as 68. I have been out there 4 times this month and it has done this every time. It never drops on initial startup or on the first 2-4 runs, but towards the end of the day it does it. Any idea what can cause this? I don't want to get on plane with it that cold because Im afraid I might seize a piston. So I end up going about 1000 rpm back to the ramp.