I have a 1998 Harley Sportster 1200. It's at the shop and they cannot figure out whats wrong with it. the bike starts up runs for 15 mins and it turns off and has no spark when until its cools down for about 15 mins. then starts back up and runs for a few more mins then turns back off with no spark. they have changed the ignition module and the coil and still does the same thing. they are lost... they have changed the petcock to a non-vacuum petcock because the old one was bad. Can someone help?