Hi I am new to this stuff, so bear with me. My 98 1998 Polaris 600 XC has a problem of going through cdi boxes. It starts with the machine idling on one cylinder and then after you get down the trail a couple of mile it will clean up and run great. Then after a few more trips it will only clean up at wide open throttle. Put on a new cdi box ($300.00 dollars later) and it will run good for another season or so. Dealer can't find anything wrong. Help