MySQL on its own server

Hi – because of high use on my site, I am running the front end code on one server and the MySQL database on a second server (on an internal network). The problem is that since I moved the MySQL DB to its own server, I am getting big delays when loading pages – it doesn’t happen all the time, but when it does happen it is bad – up to 20 seconds delay before the page loads. I am using WP-Cache as well. Does anyone know what might be causing this? Is it a known issue when you split WP across servers? It is quite a big price hike if I move to a single dedicated server (rather than two semi-dedicated) so I really want to see if this can be resolved before moving (yet again) to another server.

Thanks