i feel for you. OSC out of the box is sticky enough. i can't give you specific advice because i don't know what "customization" has been done.
you need someone who can read your php code and identify where the problem is. it sounds like there is something in there that wipes the strings which = meta tags, after a certain amount of time.
normally, i would suggest the OSC books. but this is a coding issue which needs to be looked into by someone good with php.
if finances are an issue, i suggest posting notices at the local colleges where they have a computer science department "need php help". most likely there is a parameter which can be changed to stop this timed disappearance [if that is actually the problem]
also, you need to look through the OSC forums or forums from the "customizer". if you are having this issue, other people must have too.
try posting your issue here
http://forums.oscommerce.com
i personally got so frustrated with OSC that i commissioned my own version of a very reliable php cart. if you/your client can not resolve this issue to your satisfaction and you are interested, i can provide this cart software to you at a very reasonable price. the developer has 24 hr. email tech support if you ever need it.
best of luck.