slow query since 3 days

Hello guys,
i have some kind of strange problem. I have a query on our sql server 2005 machine which always had a good performce. Since 3 days the performance is really bad. almost 10 seconds for the query. I have tried some things in a copy of the database. I rebuild the indexes --> no result. did checktable checkdatabase etc Now i have find out that when I delete the data of the last three days in 2 tables the query is again fast. when I put back the data in the table again the query stays fast.
Also the problem can be solved but I do not understand this.
has anyone an idea?
Lambik

I'll also suggest to see the execution plan for the query for both the occations i.e. when it's running fine and when it's running slow.

Resolving of such these problems can not be guessed except we know the query , the execution plan and data size
can you post these information

Thanx for all the replies. this morning I was thinking what I was doing for the last week.what could be the course of these slowness.
I have a couple of jobs for maintaining the database. Last week I was tuning one of these jobs.By these changes the update statistics did not run anymore. I now did a update statistics on the tables and the performance problems are gone.
thx
Lambikske