How can I tell what I'm doing wrong?

I recently helped a small test prep company add an e-commerce component to their website: https://www.carnegiepollak.com I installed WooCommerce and a couple other plugins to make it work with the theme and with Authorize.net. Every time I tested it, it worked fine.

Now, users of my site are receiving the “white screen of death” somewhat regularly — but I can’t duplicate the problem on-command, so I’m not sure what’s causing it. Obviously it’s a plugin problem, but I can’t tell which plugin. Troubleshooting by disabling them one at a time won’t work since I can’t duplicate the problem (it only seems to happen sometimes).

The only problem I can duplicate reliably is that people reach a white screen at the end of the checkout process, so they’re not sure if it went through or not. This does not usually mean that the site is completely crashed, though.

My current active plugins are:
Akismet (not a suspect)
Authorize.net Payment Gateway for WooCommerce
BackupBuddy (not a suspect)
Genesis Connect for WooCommerce
WooCommerce
WooCommerce Checkout Manager
WP Super Cache (not a suspect)

I’m not a WordPress pro, but I do have a little more than the basic knowledge. Can anyone help me get to the bottom of this? Please?? Googling and reading other topics hasn’t helped me so far, and I’m kind of desperate since customers are trying to give us money but sometimes can’t!