Commit problem in read-commited isolation level

Hi,
I have faced very strange problem in one of psotgresql query in one of the production environment. It is working fine in development and other environment.
Current value in colname = 5;Update tablename set colname = 0 where key = 18;commit , in parallel to above queries ( either vacuum or reindex of table was running)
After 10 sec following query is executed.
select colname from tablename where key = 18 ;it is returning old value i.e colname = 5.
After another few seconds select colname from tablename where key = 18 ;it is returning new value i.e colname = 5.

Isolevel level is readcommited.Is there any possibility of bug in commit in V8.1 leading to delay of commit ?
I need to provide explanation of above behavior to my customer.
Regards,