SQL Navigator hangs when browsing a schema (4334532)

SQL Navigator hangs when browsing a schema.

It seems OK when the number of objects is small.
For example, when expanding the Tables (of which there are 130), it works OK, but when expanding the Constraints (of which there are 630) it hangs.

It also hangs when executing 'select * from ' against some tables but not others, the difference being the width of the table or the number of rows in the table.
The select statement is also sometimes failing when running in SQL*Plus but not always.  So, it seems this is not purely a SQL Navigator problem.