So maybe either the redirect page is wrong, or the validation is broken...
Just some thoughts, but not a solution I'm afraid.
Typically, the way Dave has it would be ok... in other words the form processes on its own page and then when the processing is done (such as Universal Email), you then have Universal Email or DataAssist redirect to the thank you page or any other page after processing is complete. But, it is hard to find where the error is without seeing the code. We need to know what he's trying to do first.