Performance degrades until dump/restore

Hello,
I'm using PG 9.1. Data is streaming into one particularly large table (at
11 million rows currently) on a constant basis. It is pretty much all
inserts, very little updates or deletes (if any).
After a week or so, query performance on this table turns abysmal. If I
dump the db, then restore, performance turns great for a while before it
starts to bog down again in the following days.

So far, the only answers I can find by searching is to turn autovacuum on.
But it should already by on by default. The only setting that I have
modified was to increase shared buffer.

I'm not really sure where to go from here. Anybody have any suggestions on
what the problem(s) might be and how to rectify it?

Thanks!