Redirect/Submit Issue on MAC with Safari

Hi all,
We are experiencing a really weird unexplained issue with our Payment Form implementation. Basically the issue we are experiencing only occurs on MAC OSX 10.6.8 with Safari 5.1 (Build 6534.50).

The problem is this:
- The system displays the screen perfectly well
- As soon as the user clicks on the Country or Expiry Date or Payment Method drop downs it is like the form gets submitted and the user is taken to a blank form.

I should point out the user is not actually selecting a value in the list, just click on the down arrow to drop the list down.

We've logged this with the developer but they don't have the right environment to test. Furthermore we've inspected every line of the source code and we cannot see any script that could trigger this behaviour.

A) Does anyone out there have this environment and can see this behaviour on their form?
Can anyone think of any reason why this might be occurring?

It's terribly frustrating because it really doesn't seem to make a lot of sense but it's reproducible on every computer we can find that has that combination of MAC OS and Safari.

Any help would be hugely appreciated.
Josh