shopping cart says it is empty

Hi all,
I am having a problem with the shopping cart. It keeps dislaying the shopping cart is empty page. It is not consistant. It will display it occasionally when you hit the checkout button on the cart contents page. It will also do it sometimes when you click the shopping cart or checkout links in the header. Please if anyone has any insite, it would be very helpful.
Thanks,
Sean

What version of Zen Cart?

Clean install or an upgrade?

How did you install or upgrade?

What is new or changed on your server?

Have you added any add-ons?

Did this ever work?

If so, did you change anything since then?

Hi Ajeh,
I am running 1.2.5, this is the original version installed through Vendio about two years ago. I have added acouple of add ons, but I think the problem is even more recent. I have gotten a dedicated IP and have SSL with Geotrust. I made changes in both includes>config files. The most recent addons where imagehandler2 and easy-populate. I would like to upgrade to 1.3.7, but I am having some concerns about having gotten it this far only to fatally error my site. Of course there would be a back-up. Do you know of a really good upgrade walk-through. I followed one I found and I ended up having a copy of zencart 1.3.7 in the public html but the store was not upgraded.
Thank you,
Sean