2007 Sportsman 500 EFI runs fine and all of a sudden the atv shutsdown as if you turned key off.Sometimes if you shake handlebars it will start right back up.Other times afer a short stop if fires right back up and runs fine.I did find two wires that mice had chewed insulation off.But had the atv running and touched these wires together and atv still ran.(these go dignostic plug)We do a lot of river crossings also..

 

I had a lot of shorts in the main harness where it wraps around up into the headlight pod. This is where all the turning and twisting happens. Plus having mice on board doesn't help matters one bit! They love the insulation around the wires. Sounds like you need to go from the headlight pod and work your way down the harness. Could be as simple as a main headlight wiring short at the connector. Have had a few there also where bare wires would kill the engine when touching the headlight pod metal reinforcing bracket.PLUS make sure you don't have a key chain on the ignition switch(just the key)! Have had that problem too with em slightly turning the switch enough to kill it! OPT