PK index being ignored causing full table scan

We're running Oracle 11.2.0.3.

Suddenly in last few days some of update queries on one of table not picking up primary key index,

even in where clause we're searching row to be update based on primary key column.

It's performing full table scan.

So far I've done following but no help.

1) gathered stats on table/indexes

2) flushed plan

3) delete stats and gathered stats again

temp. work around:

use Index hint - it avoid full table scan but performing 'Full Scan' on Index instead of 'Unique Scan' !!

Update query is coming from Informatica.

When we perform select/update from sqlplus/other tool, it's using PK index !