I have a 92 explorer and I have burnt an exhaust valve twice. The first time in the scond cylinder on the pasanger side so I got the head redone. Now I've burnt the exhaust valve in the third cylinder on the same side, both times it is cremating the sparkplug it seems like from heat, and the rest of the cylinders are running hot it seems, but the car does not run hot. The second time happened within a month of the first. Does anyone have any ideasas to cause?