Vacuum analyze decreases speed

Hello!

We have postgresql 6.5.3
There is running a big database, the biggest table takes 1 Gigabayte
place.
We used vacuum every night, but without analyze.

So we decided, according of the suggestions of all the books , to make a
vacuum analyze.
Now, the speed is down. Not usable!
We recreated all the index. Nothing better!

We cannot update to 7.1x at a short time (means now, but is planed)
because this is a medical radiology database in heavy use. Maybe at a
later time.

What is vacuum analyze exactly doing? 

Where ist the statistic made by analyze stored? So I could just delete
it and make a normal vacuum without analyze to get the old state of the
database?

How can I redo that analyze to get the old state of the database?

I cannot give You an exact query. it doesnt matter, all queries are
slower than before.
#

Did you have experienced the same?
Any suggestions?

Thanks in advance

David