I have a 2002 Polaris Classic 500. Engine runs great, idles great, full power at take off. But at 40 mph, the engine bogs out like it is only running on one cylinder. If I slow down to about 25 mph, full power again and motor sounds fine. Then up to 35-40 mph it does it again. I changed the plugs the first time it happened and everything worked fine for about 5 miles, then it started again. I checked the "in tank" fuel filter (just a screen), and that looked very clean. Any ideas?