OK, thanks that helps, based on all you have claimed to have been done, it sounds like its on a good regularly scheduled maintenance program which is good, also a lot belt replacements in 550 hours ? I'm getting the feeling that its pointing to a fuel delivery issue. What it looks like is happening is under a load either the fuel pump is cavitating or the possibility of the fuel filter is clogged (low pressure). I have found that a lot of runnibility issues on these utility vehicles is usually from contaminated fuel systems.
Diagnosing this is relatively easy but would requite hooking up a fuel pressure gauge and then while under a load watch the pressure, if it drops tem likely you have a pump issue, which would requiring inspecting the fuel pump.
Now it could also be as easy as a clogged injector, you can get locally an additive that is sold in most chain Auto Parts stores, its made my Chervon, its "Techron", its is a Great injector cleaner and also will de-carbon the engine. I suggest as an easy fix would be to try that first, but before you do that it might be a good idea to pull fuel pump out of the fuel tank, inspect the bottom of the fuel tank for contaminants and then while there you can inspect the fuel filter, Polaris does not make the pump serviceable and if something was damaged or needed to be replaced you would have to replace the pump as an assembly, before you do that though I would recommend getting a pressure reading, pumps are just under $300 for this unit.
Let me know your thoughts or you have any further questions.