spark plug

I have 99 silverado w/5.3. Have had trouble with a spark plug cracking.
dealer cant seem to figure out why this keeps happening. replaced #6 plug three times. Had it repaced about 2 weeks ago and truck ran fine for a while then started to stumble again at 1500 rpm. has anyone ever had this problem before. dealer said they were going to call GM to see if they could help.

Thanks for any help

What part is cracking? Is it the ceramic, or is it the electrode?

What part is cracking? Is it the ceramic, or is it the electrode?
on the ceramic part