PHP and SSL - intermittant problem

Hi I have a weird problem with SSL and PHP. Sometimes when I click on next in an ordering process I've built it comes up with a server/DNS error. If I click "back" and try again it usually works. It is really intermittant and I haven't a clue why it's happening. It does not happen on normal html pages, but does on PHP pages. It also doesn't happen with http:// but only with https:// Has anyone else come across the same thing or have any ideas? Thanks Ade