400 outlander dies after running for and hour.

Took my 400 outlander out for the first run of the year. After about an hour or so it started to run poorly, not keeping revs up. I checked the air cleaner and spark plug, both seemed ok. Machine started up and ran fine for another extended period of time(2 hrs) and did not start to loose rpms again until I was heading home on the road and was wound out. Engine started to die again. Is it an overheating issue or a possible vent/vacuum problem.