700 XP Starts, Idles and accelerates fine for two seconds, then the RPMs drop, doesn't stall or run rough, just won't stay at higher RPM.

 

First thing I'd do is check fuel pump pressure.If it's holding less than 39 psi a new fuel pump could solve the problem.Plenty of these for your model on ebay or amazon.Even Polaris shows a #2204306 replacement pump kit,but expensive.If the fuel pump pressure shows ok,then the problem could be a problem in the tbap or tps wiring harness problem that many have had. Codes may or may not be thrown on these two items. You can check for any any obvious bare wires or soft feeling wires where the wiring from the sensors lead back to the main harness. Usually not a sensor problem itself,just the connector or wiring leading from each one. Better harness repair kits are offered here.https://otbpowersports.com/products/