Freezing and Error 500 on large cart orders

[egoodman]

I have a site I set up for a client and when they try to fulfill various orders the cart becomes really slow. Taking a lot of time to add items to the cart.... I see that sometimes it will drop an error 500 and then I'll try again and it works. Sometimes it will lock the entire store until you delete that order.

I put it in debug mode and nothing jumps out as far as error warnings. This is the only thing that came up:  Any suggestions? The site is m2cracing.net

(1/1) ContextErrorException

Notice: Trying to get property of non-object