Check this out also ...

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Intermittent TNS-12500 Errors on Microsoft Windows NT
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1. Increase the amount of memory available to the system either by removing unnecessary processes or increasing the amount of physical memory.

2. Increasing the amount of swap space will also temporarily help relieve the symptoms.

3. Disable OTRACE. OTRACE is a tracing feature that can cause many problems. OTRACE is enabled by default.
To disable OTRACE:

a. Stop the Oracle database.
b. Go to the orant/otrace/admin directory.
c. Delete all files with a ‘.dat’ extension.
d. Restart the Oracle database.

4. The TNS-12500 can be a result of the 'processes' parameter in the init.ora file being too low.

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HTH