This is a fatal PHP error - part of the page is output until it hits the PHP error & then it falls over.
First thing, check your /cache/ folder for error log entries. Zen Cart writes all PHP errors to /cache/ so hopefully this should have something useful in it. If not, ask your host if there any other PHP error logs that may pinpoint the culprit.
If you can't find any errors, the only clue would be where it fails - which is BEFORE it should load
so something might be wrong in the PHP code for this section of the page. The fact that it only occurs on some pages means little, just means that the error trigger is only present on some pages.
<div class="centerColumn" id="productGeneral">
Quickest way to solve this is to find the relevant PHP error log entry if you can.