FWIW I had a similar problem. I upgraded from php4 to php5, and then a large post on my blog wouldn’t display. I tried messing with memory, cpu and timeout settings, but nothing made it change. Then I found this page and made this change in my php.ini:
< ;pcre.backtrack_limit=100000
---
> pcre.backtrack_limit=1000000
Restarted apache2 and now the post displays.
FWIW,
Morgan