Hi again,
I poked around the site until that page was well and truly messed up. (As you know, it looks fine sometimes.) Then I copied and pasted the entire page code into my editor and had it check the syntax -- basically performing a local validation on the code -- a more complete validation than I did before. It found 42 errors. Many of them are minor but some are bound to cause problems.
The on-line validator at validator.w3.org gave 59 errors, about half of which I'd consider minor. It's a little pickier than my editor's. For comparison a similar page on one of my sites had 1 minor error. (It's difficult to eliminate every single one in a Zen site.)
Zen-Cart assembles pages from several places -- a wide variety of template files are called for the header, footer, body content, sidebars and so on. These templates get filled in with data from your database. These template files can be edited by the site owner. In fact they need to be to customize the appearance of the site, which I see you've done. So the next question is: who edited the template files?
Possibly you bought a template, or maybe inherited one from a previous site owner or employee? If so, then you've bought or inherited some problems. As well, I think you've added some mods to the site. These too can bring in problems if they're not coded for the version of zen cart you're using, or have themselves been modified.
You'll have to search through the template files to see where the problematic pieces are coming from and correct them there. It helps a lot if your editor can do a multi-file search so you don't have to open them one at a time.
I wish I could be more helpful, but there are too many errors for me to guess which is causing that problem. And really, they all have to be fixed eventually.
Rob