hi, i purchased a used 2004 yz85 a few months ago and have since been through 4 spark plugs. The bike before i got it was used for racing and the guy said there had never been no problems with it, but every time i ride it plug last 2-3 days then wont start, so dry n clean plug then starts but cuts out after 3-5 mins. buy new plug runs fine again for 2-3 days then same thing. ANY ADVICE ????