[ SOLVED ] Cart content not accurate - intermittent issue

[xvnate]

PHP: Version 5.2.17
Joomla: Version 2.5.8
VirtueMart: Version 2.0.8e
Jotcache: Version 3.2

We are having issues with the VirtueMart cart as I'll describe below.  I'm a Joomla and VirtueMart noob and have spent several days researching the issue.  I've just updated to the newest Jotcache version but still experiencing the issues so I'm now looking for some advice on how to proceed.  I'll describe a couple of scenarios which are experienced daily by a small percentage number of users regardless of browser type/version and yet, there really isn't a solidly reproducible case.  One of the frustrating aspects of this is that the issues are intermittent but are experienced by people that add/remove/update items in their cart.  Enough people get through the process that I believe that if they just add something to the cart and complete the transaction, there is no problem.
Scenario 1: Add an item from the catalog to the cart.  Assuming that the item appears in the cart, delete it and add another item to the cart.  In one scenario, what you will see is the original item listed in the cart with it's price but the subtotal for the cart will be based on the most recent and valid cart entry.
Scenario 2: In another scenario, there are multiple items in the cart.  You remove one of them and then the cart is shown as empty even though you know that there are remaining items.  If you force a reload of the page, you sometimes then see the remaining items that were hidden or sometimes the hidden items don't appear until you successfully add a new item to the cart later on and then everything appears.

We are using JotCache and I've spent a lot of time making sure that the none of the cart pages are being cached.  The Joomlas system cache is off.
The products themselves as well as their categories seem to be in order.

What I'm having trouble figuring out is where VM stores the temporary cart data so that I can try to track down how the cart state is being lost/missed/whatever is happening.  I'm not sure of the flow once the add to cart form is submitted.

I've looked at the session and vm_cart tables in the database but neither seem to have data that is recent, the session dates are more than 5 months old.
And finally something that I noticed when checking out the Product setup data in the VirtueMart Component Mgr:

Code Select Expand

Notice: Undefined property: stdClass::$virtuemart_category_id in /home/site/public_html/administrator/components/com_virtuemart/models/product.php on line 727
Notice: Undefined property: stdClass::$virtuemart_category_id in /home/site/public_html/administrator/components/com_virtuemart/models/product.php on line 733

Thanks in advance if anyone has any advice on how to track down the cause.  A little sleep deprived so I hope that this message makes sense!   

[K&K media production]

the cart data is stored in the joomla session from the user

[xvnate]

kkmediaproduction - thank you for the reply.  I believe that I was in a sleep deprived state and was looking at the wrong database when I was looking for recent session data when I posted my request for help.
The answer, as it turns out, was that the system cache plugin was still enabled and that had the effect of having users sharing sessions.
Thanks again for the reply.