Had that exact problem just the other day...well sounds the same. Noticed the trolling motor was lagging a bit just didn't think too much about it until it wouldn't even pull the boat in high gear after an hour or so. Drifted around for a few minutes and it would haul butt for about 5 seconds then dead again. I knew I had help because my uncle was in his boat fishing close by so I went ahead and switched my starting battery over to my trolling motor and it worked fine the rest of the day and still had enough juice to start the big motor. Someone else mentioned the load.. There is a difference between your volts at rest and your volts while under load. I don't understand it so am not going to try to act like I do, but I know there is a difference.... my guess is your battery or batteries are done for.....I don't know if there is any one battery brand better than the next as I've had them both go bad and both last.