slow seqscan

I have already posted a similar topic months ago, that time I had a
strange behaviour checking response time of seqscan after a vacuum, but
now has happened again in another situation.
Scenario:
I have a database used only with search queries with only one table that
holds about 450.000/500.000 records.
The table is well indexed so that most of the queries are executed with
index scan but since there is a big text field in the table (360chars)
some search operation ends up with seq scans.
This table is not written during normal operation: twice per week there
is a batch program that insert about 35.000 records and updates another
40.000.

this morning, after that batch has been executed, the database started
responding really slowly to seq scan  queries, also after a "vacuum full
analize" things didn't get better.

I am using version 7.3 do I need to upgrade to 7.4? also, I was thinking
about setting this table in a kind of  "read-only" mode to improve
performance, is this possible?

Thank you for your help
Edoardo Ceccarelli