Is that related to the missing post parameters? It’s true that with a large mashup the amount of data can be fairly large – maybe a worst case of 1k per post – but a few hundred posts shouldn’t cause a timeout.
Maybe the back-end is running very slow. The fact that it’s intermittent might indicate a lot of users on your site (or someone else’s, if it’s a shared server).
If you think that’s the problem you can use posts_per_page to limit the mashup size.