See post on "child exit - bus error'.
I am experiencing the same thing, but the messages in the apache log USED to be segmentation faults, now they are bus errors (in my case).
I've been debugging the code forever only to find that the code really does not work at random, like in your case.
After looking on the web, I've found some other people who are having the same errors, but mostly with Solaris. Since you don't have Solaris (and neither do I - I use HP-UX), the problem is not OS related.
Sorry I didn't have the solution.
Tim