unpredictable behavior

I am running postgresql  7.3 on fedora core1. The database encoding is
euc-jp.
I have unit tests that test my persistence code what is troubling is
that the tests fail with the exception listed below but if I rerun the
same tests they pass. Out of 10 runs, 8 would pass and 2 would fail.
Any idea why the behavior is not consistent? I would rather that they
always fail.

 End of Stack Trace
    [junit] ]; nested exception is:
    [junit]     Something unusual has occured to cause the driver to
fail. Please report this exception: Exception: java.lang.
IllegalArgumentException
    [junit] Stack Trace:

    [junit] java.lang.IllegalArgumentException
    [junit]     at java.nio.Buffer.position(Buffer.java:218)
    [junit]     at java.nio.charset.CharsetDecoder.decode
(CharsetDecoder.java:575)
    [junit]     at java.lang.StringCoding$CharsetSD.decode(StringCoding.
java:186)
    [junit]     at java.lang.StringCoding.decode(StringCoding.java:224)
    [junit]     at java.lang.StringCoding.decode(StringCoding.java:230)
    [junit]     at java.lang.String.<init>(String.java:371)
    [junit]     at org.postgresql.jdbc1.AbstractJdbc1Connection.
openConnection(AbstractJdbc1Connection.java:226)
    [junit]     at org.postgresql.Driver.connect(Driver.java:122)