Sputtering issues on '06 400 4x4 auto

I'm a long time viewer first time poster, I have a 2006 400 4x4 auto quad that has been sitting for about 8 months. It started up just fine and I rode it around a bit. Then I left it overnight and started it up again and it idled just fine. As soon as I put it in gear it started to bog down once It passed first gear as well as in reverse, but it never stalled out. I drained the tank and put in fresh gas, checked the plug and seafoamed it, still the same thing. Today I noticed that with the reverse over-ride switch on it runs in reverse just fine, and I also noticed that when I lightly grasp the handle brake, it runs perfect and does not bog down or sputter. Does anyone have any clue as to why this would happen?