Honeywell Pir Is2535 False Alarm

I put new system in 04/2011, in the last 2 weeks, I had 3 false alam in my Basement (is emty and clean).  the firts time was at 7am, when was activated my system I jump from my bed, and permit that my company call the police, after the check the basement every thing was fine, the second time was in the same day in the afternoon. I call a mantinance service ($90 for the first 30min) from my company, they check, test, etc, every thing ok, but the person said that he can replace by $102, I told him the sensor cost less than $30....no way....4 days later again a false alarm, all of them in basement....what happend???....