Internal Server Error with full excerpts

We have a catlist shortcode that returns more than 300 posts from a category. If we leave out the excerpt or use excerpt=yes, it works fine. If we use excerpt=full, it returns an internal server error even with 512M of PHP memory, execution timeout of 180 seconds and 64MB post size. There are manual excerpts on most of these posts. Is there anything that might be in an excerpt that could break list category posts like this that I can search for? Have you seen this happen before? Is it just a size thing where that many full excerpts is too much content for memory, for sql query, or something else?