oops
sorry, that was the wrong advice for your problem, as the action should be left blank since it is posting to itself.
Try this instead:
Go to about line 34 or so . . . you are looking for the:
//Send Mail All Entries
if ("index.php" != "") {
header("Location: thankyou.php");
}
make sure that the location is to the thank you page.
What is the code you have for your flash banner/menu? That may ultimately be the problem. Post that code if possible.