The problem is that unless someone has encountered your exact scenario--which is quite unlikely--the only way to find a solution would be to try a few things out.

But in order to try things out, anyone considering doing so would have to try and reproduce your code from your description. And that's a hard thing to do and rather expecting to much of us. cdarke had a good attempt, but fell short.

If you want further help, you're much more likely to get it if you would post a minimal, runnable example that can be downloaded and tried out. Perhaps you could start with cdarke's example and modify it to reflect your requirements to the point where it produces the error message your getting.