A racer buddy of mine "civic91", had the same problem with his Sorex tires. He said they would actually chunk at the spot where the inner belt overlaps itself. Rubber tires chunking! Thats not good.
The funny thing is that I have never used a tire that was factory belted. And I actually stopped belting my own tires years ago. Just didn't seem neccesary with the traction I was getting with the Losi lowprofile Yellows. Maybe I will try some belted tires next.