AbleCommerce Shopping Cart Software

My customers are complaining that they are getting strange checkout problems. One got to check out but couldn't complete the order, it stopped processing. Another said pages are missing and couldn't get back to the checkout. I haven't been able to reproduce any of these issues on IE 8 or FF 3.5. I was able to checkout out and process the order, but if I clicked on another part of the page, it doesn't redirect me to http://www.site.com instead it redirects me to https://www.site.com which results in blank pages. I'm not sure these are related, but I did discover that one of the customers was using IE although I can't verify the version. Any suggestions why this is happening?

As discussed in another thread, the blank pages problem is an IE 7 issue on Windows 2003 Server. I was able to reproduce this. My customers are still complaining about the "processing..." box showing up. The order is never recorded. This seems to happen to some customers and not others. This leads me to believe the issue may be either on the server or their web browser.