Long drives then acts up and dies

Hey everyone, havent owned my 2012 850xp for very long and although its a used bike its been great so far until the other day. We went for a long drive about 3 or 4 hours in my bike started to die, started to get low power then eventually no power and would just die out. If i gave it more gas it would just backfire. The weird thing is i could start it up and all my powers back and works great till i get going again then same thing dies out. So what i did to get it home was just ease on the throttle to speed up and once i was going a decent speed kept it there and i had no issues.

The next day i took it out for a spin and all was great. like nothing happened, so we went for a decent drive and yet again after a few hours it started to work like crap.

All i have done so far is replaced the spark plugs, which were pretty black. Havent gone on a big run yet with the new plugs because we got another snow storm and the snows getting to deep now.

If you guys have any suggestions on what it could be that would be great. Would rather fix it now while the snows here so it can be operational for the spring.