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,