Sluggish 2008 Optimax 135 operations

I have a 2008 Optimax 135 HP on my 21' triple tube pontoon with about 195 hours. At the end of last season it starting operating sluggishly. When newer, following the proper breakin, max RPM with a 15 pitch Merc SS prop was 5220 at 35 mph on a lightly loaded boat . Now I can't even get 5000 RPM trimming the motor up to nearly cavitation. It used to jump out of the hole but now it's very subdued. I tried changing the plugs at 175 hours to no avail. I have always used top quality gas and change the fuel and air filters. I've never found any water in the fuel filter. I operate on frest water and the fuel comes from a gas station. I've never used any fuel additives. I'm using the Optimax oil. Much of the boat operation is slow cruising at low RPM (~1400). It acts as if it's dropping a cylinder and occasionaly acts like it wants to pick it up and go but then drops back to poor performance. Fuel usage appears to have increased at the same time. None of the plugs looked different than the others. It's still under warranty but the dealer is 90 miles away and they screwed up the installation of the console gages so ... Is it possible the injectors are fouling? Would it be worth running some injector cleaner? Any suggestions?
Thanks for any help!