Take a look at the clutch nut before you stick a new set of bearings on it...If it is worn, it will eat the new bearings in a New York minute.
When you put the clutch together, do you put the shoes in leading, or trailing? I always put mine in so they engage with the leading edge, but I'm just running 2 shoes with a spring. If they are put in trailing, it is conceivable that they slip enough to generate enough heat to melt the spur. One time I put a set of trimmed stock shoes in backwards like that, just to see. I knew immediately that it wasn't right, and it blued the bell in just a couple of laps around my track.

http://www.youtube.com/watch?v=iuszkqne0Mk
Shameless plug for my new vid of my track, and my new puppy(not so new anymore, I got him this spring)