Whats the clearances on your cam bearings? Having oversize bearings on one journal will cause the cam to deflect and cause the lobes in between the two good journals to wear unevenly and eventually wipe the lobes. This is a very uncommon condition and is usually caused by the cam bearings being replaced with the wrong size at one journal position. Other than that there is little else that would cause this, you could check for this by comparing lift at the valves across all the cylinders with a good cam. If you find a few cylinders with low lift (0.003" or more) you will find the journal that is oversize. This is a good reason to check the diameter of every cam journal bearing before installation. This problem is hard to find because it only gets bad at rpm, if you were racing it you would probably break cams in half.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo