[site is acting strange] - blank pages, download index.php, or works fine

I have the strangest issue with my host. They can't figure it out and I'm
completely perplexed. We have other sites running on the server just fine.
However, this new site is acting very weird. Sometimes we get blank pages,
sometimes we get a blank page and then a dialog pops up asking if we want to
download index.php, and then sometimes the site is working fine. Any ideas
on this?

I'm at ends. Appreciate any advice. For authentication we are using mysql
auth module in apache/linux and proxy pass. We removed proxy pass to see if
that was it but, it wasn't. its a members.domain.com subdomain if that
helps.

Thanks ahead, TS