Marketpress - shopping cart issues

I have a site that recently began having a problem w/ the shopping cart - when you choose a product to place in the cart it shows in the left sidebar shopping cart but when you go to checkout the cart from this window the cart is shown as empty - it doesn't matter if cookies are allowed or not or if you refresh the page - BUT if you go to another page on the site and then go to checkout the product is there - It is causing visitors much confusion.
Help Please!
Currently we are on version Version 2.5.9 of Marketpress and WP 3.4.2 - I have been very fearful of doing the upgrade of the Marketpress - since the client has many products and can not tolerate site interruption.
Is my upgrade phobia warranted?