TNS-12451 - but only intermittently(!)

[BigGee]

We have 2 servers that are experiencing an intermittent tns-12451 error, one of the servers is in the DMZ, one in the main LAN, most of the time the DMZ server can connect to the database server on the main LAN without issues, however approximately every 5 hours the server in the DMZ can't connect to the main database server, reporting the error as TNS-12451.

During the time of the problems, you can connect to the database on the database server itself via SQLPlus no problem, but if you try and connect to the same database using SQLPlus, but from the serevr in the DMZ, you get the issue.

After 15-20minutes the issue goes away and normal service is resumed, despite no action being taken.

Both servers are running oracle10g r1.

I'm not even convinced this is an oracle issue, given that it's only intermittent but our network admin says he can't see anything amiss so I thought I'd ask on here in case anyone has experienced anything similar before.

SQLnet.log on the server in the DMZ contains entries like the one below when trying to connect to the database during the problem times.
VERSION INFORMATION:
TNS for 32-bit Windows: Version 10.1.0.5.0 - Production
Oracle Bequeath NT Protocol Adapter for 32-bit Windows: Version 10.1.0.5.0 - Production
Windows NT TCP/IP NT Protocol Adapter for 32-bit Windows: Version 10.1.0.5.0 - Production
Time: 12-MAR-2009 23:15:54
Tracing not turned on.
Tns error struct:
ns main err code: 12541
TNS-12541: TNS:no listener
ns secondary err code: 12560
nt main err code: 511
TNS-00511: No listener
nt secondary err code: 61
nt OS err code: 0

Some days (though it hasn't happened for 4 days) we also have entries such as the one below when the problem occurs - today the issue happened and SQLnet.log only contained the tns-12541 errror.

VERSION INFORMATION:
TNS for 32-bit Windows: Version 10.1.0.5.0 - Production
Oracle Bequeath NT Protocol Adapter for 32-bit Windows: Version 10.1.0.5.0 - Production
Windows NT TCP/IP NT Protocol Adapter for 32-bit Windows: Version 10.1.0.5.0 - Production
Time: 02-MAR-2009 13:33:28
Tracing not turned on.
Tns error struct:
ns main err code: 12537
TNS-12537: TNS:connection closed
ns secondary err code: 12560
nt main err code: 507
TNS-00507: Connection closed
nt secondary err code: 0
nt OS err code: 0

I've checked other posts on this subject but they all seem to be based on the issue ALWAYS happening, my issue is only intermittent, hence a new post on what must be a "classic" subject (for which I apologise)

Note that the alert.log and listener.log files both look normal, no obvious error messages,

Can anyone please offer a nudge in the right direction?

[PAVB]

In my experience that kind of scenario points to a networking issue... I would start at OSI Layer 1 meaning, start by checking the network cables, replace them if in doubt then work your way up thru your network.