Dual NICs for Internet/Intranet Sharing

I have a client that we are using an XP Pro SP1 machine as a Proxy Server
(not MS Proxy). This PC has a NIC that is connected to their Intranet for
sharing the Internet and a NIC that connects the Proxy Server to a
Router/Firewall, which in turn is connected to the DSL Modem. This has
worked fine up until a couple of weeks ago (was fine for 3 or more months,
also running this scenario in several other offices, including my own, for
several years). Lately the internet will stop responding on the Proxy, if we
disable the Intranet the Proxy then can access the Internet (but of course
no-one else can). If we do a System Restore (one of the only systems I have
seen this work on) back to a day is was working, it will work fine again for
a day or two, then right back to the problem, again! BTW we have Windows
Update Turned Off.

Thanks in advance for any help in this matter!!!
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