spark plug question

[julsco2000]

I have a 1974 Z 50 that continues to kill spark plugs. I am using NGK cr6hsa plugs. They will work for about 1/2 hour to 1 hour and then die. The plug will spark across from the threads to the head but once you actually touch the plug on the head there is no spark across the electrode like there should be. Put a new one there and it works fine for a short period. I have a #58 main jet and all seems fine with the carb. Checked the piston rings(thought there may be oil ring problem) all is good. Any ideas???