performance issue running full text queries

Hi:
I'm having an issue with SQL server's full text index
we have a full text index query that runs every 20 minutes and the performance sucks(like I need to wait like 6 minutes for it)
but if I run the query immediately after the first run(that is rather than run it every 20 minutes, I run it every 1 minutes) the second run is good(finish in 2 s)
I figure out the problem lies in the full text catalog(which is very big) being kick off the cache and then when it re-run, the catalog need to reload to the cache, do u have any solution for this?
add memory?
thanks

I followed all the suggestions given in the aboved link
and the problem still exist
my dedicated server have 2G memory
I set the cache size in the system to be 7G
the db cataklog is about 700M
DB is about 7G in different drive
I set the max server cache to be 1200MB in SQL server
but that query is still slow if I run it every 20 minutes
so I guess my question is is there any way of always keeping the full text catalog in cache?
Thank u very much

http://support.microsoft.com/?kbid=915850

http://support.microsoft.com/?kbid=915850
You can get poor performance if the server doesnt have internet access