certain customers not able to view cart after upgrading

I have recently had a company upgrade my cart from 1.3.7 to 1.3.9a about a week ago. In the last week since its been installed, I have had several customers get to the checkout process and cannot see the cart. I have logged in numerous times on ie 8, safari, fire fox and I do not have any problems. We have noticed that our web sales are down which is alarming in its self. The things that I know for sure are that my paypal IPN doesn't recognize coupon codes even though the invoice is correct in zen cart. Also when I try and click on the send e-mail link under tools in admin, it hangs and doesn't ever get to where I can send e-mails. After that, mysql resources are redlining at 98% and if you're on my site trying to checkout, it will not show the go to checkout screen, I have to restart the server in order for it to back down and resume to normal service. Also if I move an order from processing to another folder that I added called ship later, it goes to error 404 but still moves it to the folder its just that I have to back out of it to get back to admin. The person who upgraded the site is out of the country on vacation and will not be back for another week and half and I want to see if someone has any ideas on what could be happening. my url is https://www.lowcarbyou.com

thanks volks

Sounds like some things were missed during the upgrade. Unfortunately those "things" are not easily identified without a careful examination of all the files on your site ... which is what the person who did the upgrade will have the best knowledge of.

In the meantime, are there any errors being logged in the myDebug-xxxxx.log files in your /cache/ folder?

Additionally, v1.3.9b has been released, and contains several updates to a handful of PHP files, to address a few reported issues.