sounds like you have some corrupt files uploaded to your server, or else you have some modules installed that aren't playing friendly.

I'd try:
- removing all modules that didn't come with Zen Cart
- re-upload original files after re-unzipping them. Use a reliable FTP program like FileZilla (and not SmartFTP or cuteFTP)

If all else fails, it may be necessary to do some harsh diagnosis:
http://www.zen-cart.com/wiki/index.p...Obscure_Issues