Do you have solid lifters? If so, you may have too little clearance. When it warms up, the gap gets smaller and you blow compression by the valves. If this is the case, make sure you adjust them, because this is what burns valves. Also, is your gas fresh. Stale gas will cause valves to stick. Have the valves been reworked lately? If there is not enough clearance in the stems, the valves will stick when they warm up.
