Confirm Order Button Not Working Correctly - Need to Diagnose and Fix - ZEN CART

We've been getting a lot of customers bailing and a lot of them write with the same issue. Here is a typical customer email: "I entered all details and clicked to confirm order. This seemed to refresh the page, but just back to the same page. I clicked confirm order again, which did the same thing. I then panicked that the order may then have gone through twice so decided to come away from the page and see await confirmation emails. Nothing!" We get quite a lot of these messages and we've tried to replicate the issue. It seems to be intermittent as sometimes we can replicate it and sometimes we can't. Here is one method we've used to replicate the issue, several times: Here's the steps I took to replicate the problem: 1. Add a one penny item to my cart, log in and go to checkout confirmation page. 2. Deliberately select "Visa" from the drop down and enter a "Visa Debit" card details. 3. Receive the red error message informing me that I should have selected Visa Debit. 4. Correct drop down to "Visa Debit" and re-enter card details. 5. Click "Confirm Order" button but the order WILL NOT confirm. Instead it just semi-refreshes and jumps to the top of the page! 6. Look at it again and you see it has re-selected the "Visa" from the drop down menu! 7. Re-select "Visa Debit" and enter card details again. This time it goes through when I click confirm order button. However, customers are reporting that it's intermittent and it can take three or four attemtps.... lost of them are just bailing. There are other ways to cause the same problem. For example, enter a coupon code and click the refresh botton to apply the code. This will change the order total but when card details are entered and the confirm order button is pressed, it will jump back to the top of the page. You'll need to click the confirm order button a few times for it to go through! The same problem occurs when you select a different shipping option using the radio buttons.... Can anyone diagnose this problem and fix it for me?