I can't be sure about this, BUT...

It sounds like it's running on 1 cylinder.
It is quite possibly the Engine Control Module. (Computer) (About $300)
Under the drivers seat, in front of the storage box.

I had one go out, and what you are describing sounds very familiar.
Mine started running that way, and never went back though. (Not something you want to happen 50 miles from the trailhead)

Get that thing checked out before it leaves you stranded.

Yeah, there were some tremendous upgrades done in 2009. If you drive one, and then the other, you will find it hard to believe they are even related.

Depending on several factors like elevation and the kind of terrain you will be riding in, I think the 500 would be way underpowered for 5 passengers.
I ride at elevations between 3,000 and 10,000 feet, and even sometimes found the 800 underpowered with 5 passengers. Sand dunes & high elevations are the worst situations. The 500 doesn't come with the new style suspension either.

With 5 passengers I found myself in low range struggling to climb out of some the bigger bowls at Little Sahara sand dunes. (About 4,000 feet elevation) Even the 800 needed to be in low range, or possibly have some serious clutch work done, because I just couldn’t get the RPMs up. It was bogging down.

The Ranger XP has some real disadvantages if you plan on putting a seat in the back, filling it with kids, and climbing any kind of steep inclines. It is Tippy! Sand dunes, Moab style obstacles, or even steep dirt trails can be sketchy.

Because I have kids to haul around, if I had it to do over I would have started with a 2010 Ranger Crew. The changes made in 2009 on the Ranger XP finally made it into the Ranger crew in 2010 (Along with the 800 EFI engine) The Crew also has lower gearing that multiplies the available torque better than the XP, so it’s like getting the 4.10 gears instead of 3.42 gears in your truck.

Anybody in Utah interested in loadin’ up some camping gear on your UTV & getting into the back country this summer?
I just bought an ATV wagon to stow my camping gear in, and want to go explore some places like the Amasa valley trail system, & the lakes in the Uintah mountains.
I am thinking about 1 and 2 night weekend trips throughout the summer.

Anyone interested???

ATV wagon.
http://www.youtube.com/watch?v=uTfXvk0Ll4A

Check these videos of the Amasa valley out...
http://www.youtube.com/watch?v=k6ub0c532Xk
http://www.youtube.com/watch?v=UxxKBuT8G44

WELL DONE!!!

Thank you.
It's all for play.
Problem is, just when I get it done we find out we have a new addition to the family on the way. There will soon be 7 of us!
Now I need to sell it and get a Crew!

I would be interested in taking the travel trailers to Moab and doing some dry camping/riding out in the general area where we went over Presidents day 2009.
It should be a bit warmer in late March, than it was in Mid Feb. (Maybe the weekend of the 26th?)
Is something like that what you have in mind Lynn?

How about posting the big news… when you have it all done.  Why the suspense???