Ok I lost my original rate when I close out accidentally, so this rate will be shorter. I agree with Raseri and especially ium's changes wholeheartedly. They have already made most of the suggestions I would have, but there is one aspect that you should look at: Wartortle.
I've tested your team out with the above changes, and I've seen one major problem that is prevalent throughout all my battles: Wartortle is dead weight most of the time. While spinning is great to preserve Charizard, Roost and Leftovers already go a long way to keeping it healthy. Out of the rest of your team, only Volbeat takes a lot of damage from Rocks, and Spikes would be more problematic is your main sweepers didn't all have some form of recovery. Wartorle's Scald is rather useless in the Sun since it doesn't even 3HKO Golurk anymore (0 SpAtk Wartortle Scald vs 0 HP/0 SpDef Golurk: 23.51% - 28.21% (4-5 hits to KO in Sun)) and fails to always break Haunter's Sub (0 SpAtk Wartortle Scald vs 4 HP/0 SpDef Haunter: 21.98% - 25.86% (4-5 hits to KO in Sun)).
My main recommendation is to replace Wartortle with something that'll maintain offensive momentum. I've tested a few different options, but I like Choice Band Sawk
the most. With Mold Breaker, it keeps offensive pressure on your opponent's team even outside of Sun, while detering many Rock types from setting up early in the game. Another option would be to bring Gardevoir back as a Life Orb attacker with Destiny Bond/Healing Wish, but that option is more to prevent changing too many members of your original team than anything else. It DOES give you a reliable check to opposing weather teams, as well as a way to weaken Skuntank before Exeggutor has to face it.
Choice Band Sawk>Wartortle
Life Orb Gardevoir>Wartortle
Admittedly, ium's changes are all you realy need for this team to at least function well, but Raseri's change is very helpful to really use the Sun properly. My suggestion(s) just adds on to the theme of Sun abuse while still trying to maintain a utility role for your team. Good luck with testing your team!