Strange problem with an index.

I have the following table

Table "public.authorsort"
Column | Type | Modifiers
----------+-----------------------+-----------
ordernum | integer |
handle | character varying(30) |
Indexes: author_handle_idx btree (handle)

As you can tell I have an index on handle.

In my perl script I loop trough series of select statements that look
like this:

Select ordernum from authorsort where handle = 'somvevalue';

Well, this was running very slowly, so I droped the index and recreated
it, and then it ran quickly. I'm running versin 7.3 and I noticed that
in versin 8, the following has been done:

Prevent index corruption when a transaction inserts rows and then aborts
close to the end of a concurrent VACUUM on the same table (Tom)

And we recently added vacumm --full to our system. Could this be the
reason?

Thanks!
Jose