Have you also set ORACLE_HOME and ORACLE_SID environmental variables?
Can you PING the server using the same value you have on TNSNAMES.ora file?
Can you TNSPING the instance if you fully qualify the tnsping call? meaning, not relying on tnsnames.ora file?
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Pablo (Paul) Berzukov
Author of Understanding Database Administration available at Amazon and other bookstores.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.