Problems
Due to the lack of a Fire-type on your team, you have a notable weakness to opposing Grass-types, especially offensive variants of Shaymin. It has enough power and type coverage to 2HKO the majority of your team, and outspeeds many team members as well. Currently, your best way to take it down is to hit it with a super-effective move from the likes of Jirachi or Rotom. However, Shaymin is bulky enough to take a Fire Punch or Hidden Power Ice without much of a problem, allowing it to hit back hard with Earth Power or Seed Flare, respectively. To make matters worse, Shaymin has easy access to full health recovery with Rest and Natural Cure, meaning that it can be rather difficult to defeat. It also has an easy switch-in opportunity on Swampert, which will require you to switch around your own Pokemon to avoid being KO'd. Regardless, a well-played Shaymin could potentially sweep through your team or at least cause you to accumulate a large amount of residual damage.
Suggestions
I feel that Swampert is the most easily replaceable member on your team, and could be switched out for something that would fix up your Grass weakness. Swampert's main role on this team is to function as a bulky user of Stealth Rock. Therefore, I'd like to suggest using either a Passho or Shuca Heatran in its place. Heatran's unique typing will help out a lot against Shaymin (and Celebi to a lesser extent). It also synergizes well with Machamp in terms of lead matchups, as the two can beat several common leads when used in conjunction. However, this does result in a large number of Ground weaknesses on your team; Shuca Berry will fix this issue somewhat, but Passho allows you to survive a LO Hydro Pump from Starmie and KO back with Explosion. If the prevalent Ground weaknesses get to become too much of a problem, additional changes may be helpful, but it's up to you find the best choices from testing.
The only problem I can see with replacing Swampert is that it opens up a weakness to opposing Dragon Dancers such as Tyranitar and Dragonite. One option to deal with this would be to try out a Scarf Jirachi over your current set, using the moves Iron Head, Fire Punch, Ice Punch, and Thunderpunch / Trick. Jirachi is one of the best revenge killers in OU, and it gives you a way to deal with several different threats. In the last moveslot, Thunderpunch is for revenge killing Gyarados, but if you feel that you have it reasonably well-covered with Rotom, Trick is a good alternative option for its ability to cripple stall team members and bulky stat-uppers like Suicune.
Good luck with your team!