I have a DB running on a sun Server, and Web Servers running Win2K/IIS5.0 with Oracle client 8.1.7 connecting to the DB. this system has been configured and running for a while.

Everything's working fine, except that at some points, usually during high load, lots of current connections per web server, the web servers seem not to be able to access the DB.
sqlnet.log on a web server will show an error like the one below. The server's listener log file shows no errors, I actually notice that during these periods the server has a light load.
I had network people checking the network layer and there doesn't seem to be any TCP/IP level error, all ports full at 100Mbs full duplex

all machines are behind a firewall. again, there has been no change to the network layout or configuration.

---sample errors from sqlnet.log on the client ----
Fatal NI connect error 12535, connecting to:

TNS for 32-bit Windows: Version - Production
Oracle Bequeath NT Protocol Adapter for 32-bit Windows: Version - Production
Windows NT TCP/IP NT Protocol Adapter for 32-bit Windows: Version - Production
Time: 12-OCT-2005 22:15:58
Tracing not turned on.
Tns error struct:
nr err code: 0
ns main err code: 12535
TNS-12535: TNSperation timed out
ns secondary err code: 12560
nt main err code: 505
TNS-00505: Operation timed out
nt secondary err code: 60
nt OS err code: 0