1 TNS error out of 25 connections

HI I have a strange behaviour of my remote Apache-PHP-Oracle-Client. Apache 1.3.14 mod SSL PHP 4.0.3pl1 (everything compiled from sources) Oracle 8.1.6 on remote Intel/Redhat 6.2 (100MBit, same subnet) The Oracle Server runs a Multi-Threaded-Server (successfully) and accepts dedicated Serverconnections on the same Port. tnsnames.ora and sqlnet.ora are placed in the proper directories. it works in 95% of all requests, but sometimes (with no specific traffic reason or network problem) it shows an typical _oci_openserver_error: COULD NOT RESOLVE SERVICE NAME. I looked up Oracles Online Help but it's just like check your tnsnames.ora, check your servicename ... But as it works in 95% of all cases, I do not think that the problem is my path to tnsnames.ora Is there anyone that is familiar with that problem or the configuration of MTS and Dedicated at the same port / time ? thank you mk