hydro shield?

[Bushwacker]

I think Gillie and Ken nailed it with the fuel sloshing/tank baffle comments! 120 gallons is a BIG tank that should have several baffles in it.

When a boat is already marginally stable in the roll axis, an un-baffled tank would tend to make it highly unstable because a little bit of roll shifts fuel weight in that direction, causing more weight shift which causes more roll, etc. A total of 160 gallons of fuel that's free to go where ever it wants to is almost a half a ton of mass that you have absolutely no control over! A 400 lb cooler in the cabin is a nit by comparison! Imagine having a roly poly 500 lb passenger free to roll around on the deck that immediately rolls to the low side as soon as you start a turn! I'm surprised you're even able to control the boat at all! Kinda like trying to balance an anvil on a broomstick!

Did you notice any difference in handling between when the tanks were completely full and maybe after you burned off ~ 1/4 tank? The mass can't move around when tanks are completely full, so I could see you starting off the day with the boat handling fine and then gradually turning in to a nightmare!