Please help ! My Mariner loves to go fishing but wont come home ! (1997 25hp )

Hello and welcome to my nightmare! I have a 1997 25hp merc mariner that I bought as a package on a 15' Sylvan Deep V Aluminium bass boat.. I bought it used with about ( According to seller) 10 -15 hrs on it . I have taking for repair to local repair shops in my area 3 times now , each time problem solved but new ones are always just a few trips away!! Very frustrating ! Since ive owned it Ive replaced the CDI , Changed prop, Had the Carb Rebuilt , a new impeller for the cooling and a new thermostat All in the last 3-5 years.. The boat in past years hasn't gotten allot of use a couple trips a year.. <br><br>So heres my current situation.. In June I was on a 5 day fishing trip (Fresh Water, Never been in salt water) and the boat worked perfectly! No Problems! So a couple weeks ago we decided we would see if the 25hp would be enough to tow the kids on a tube. Put the boat in the water the night before to it for a few laps around the lake and it worked great.. Next morning I tried to tow a tube , No luck just not enough power to get any speed up. After about 20 minutes of trying different motor mount setting the motor began acting up . It was definatly under load as it would only do 10 mph towing a tub and when the tube dipped down the boat would almost stop .. After the motor was under considerable load it started acting up, Misfiring , fouling plugs and eventually lost the top cylinder... I cleaned plugs , and managed to limp back to shore.. After checking fuel , spark, wires I determined the next step was to replace the coil to the top cylinder.. I did so and took the boat out for a spin just me and my neighbour, Motor worked great ! So the next morning 3 of us loaded up the boat with our fishing gear and headed out for a fish.. We travelled to the opposite end of the lake full throttle about 18mph with no problem ! Plenty of time to heat up the motor.. We shut it off used the trolling motor and fished for about 4 hours. It was time to head back so away we went and within 50 to 100 yards it started misfiring and acting up again , we crawled back to shore , She would die at any more then half throttle with the bottom cylinder in and out.. So since i bought 2 coils I replaced the 2nd coil and took her for a spin ( Just 2 of us ) .. Again it tested fine took about 10-15 laps around the lake Full throttle with no problems... The next day we loaded back up and went for a fish.. Same thing worked fine from shore to fishing spot ! Shut off and upon time to head back a few hours of sitting it immediately starting misfiring , dropping a cylinder , picking back up but wouldnt run full throttle and we limped back to shore again... I have fresh gas and doesnt seem to be any fuel issues ! Please help I want to go fishing again !!!!!!!!!!!!<br> <br>Any suggestions before I take it back in for $$$$$$$ Service ! Am I missing something ?? <br><br>