Caching nameserver and odd MX problem

[Jason]

my MAILserver
mail.cartmanager.net
is haveing a bugger of a time sending mail to this domain name
prepaid-phonecards.com

It functions perfectly until the cache for the MX record expires, then will not send any futher mail until I restart my nameserver
Here is a snip of the sendmail reason for failure
Deferred: Name server: prepaid-phonecards.com.: host name lookup failure

my nameserver is
ns.cartmanager.net

I am a little new to detective work on the DNS front, but have read several documents in the list archives.  And have found may
POSSIBILITIES at to why the domain name may be having trouble.

So far I think I have ruled out many of the list archive suggestions using the "dig" tool.  However, I still cannot find why it is
failing...

In fact the only thing I have not been able to rule out is that the MX record appears to have a different cache setting then the
rest of the prepaid-phonecards.com domain name.  My only guess is that my nameserver (named 8.2.3-REL Mon Jan 29 06:04:52 CET 2001)
is not looking up the MX record due to the fact that it still has other information cached for that domain name.

Is there a way for me to track down what may be wrong.  Its really annoying to have to restart my nameserver (even though I could
crontab it easily)

And how do I fix it?


Thanks
    -Jason