1) What is your server and db versions?

Slackware 7.1 and Oracle 8.1.6 Server.

2) What is the client server and db version?
Windows NT4 with Oracle 8.1.5 Client.

3) What have you used in the tnsnames.ora service_name / sid ?

TEST =
(DESCRIPTION =
(SOURCE_ROUTE = OFF)
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = ipaddress) (PORT = 1521)))
(CONNECT_DATA =
(SID = TEST)
)
)

4) Is there any timeout set in the sqlnet.ora?
SQLNET.EXPIRE_TIME = 0