A Merc that trims up by it self is usually the trim switch in the control handle and the fastest way for it to short out is leave it out in the rain for lond periods of time.....see pic above, any way most Bass Boats have 2-3 trim switches one in the bow, at the control and some where around the transom, and finding the on acting up can give you fits, first you have to get it to do it, then unhook the switches one at a time to find the bad one, all the while keeping in mind it is doing this because the switch is shorted out and unhooking the battery will make the problem go away most times until when you least expect it , it runs on it's own again. I would access the back of the bow switch and the one around the transom if it has one, when the problem starts , stop the boat, unhook the read wire one at a time with the 2 switches and see if problem stops, both unhooked and still running it's the control switch and the handle has to come off to replace it.
Tom
"The reason I play Golf.....there are no broke down boats on a Golf Course"
Well, the deal was done. I was on my way to Alabama to buy that boat in the morning, and he sold it out from under me. Its OK though. He only wanted $2650. for that boat. Dadgummit. The good lord works in mysterious ways. Something better is coming my way. Thanks for all your input.