mold-only in Fall,

I have a strong, biting mold smell that comes from the basement, but only in the fall. Never in the winter summer or spring. It seems to last 6 to 8 week and goes away until next fall. It seems to be worst just before a storm.
I have never found were it comes from. I have used a strong light to look for it.
I have tried using a pump sprayer with bleach and water, sprayed walls, floor and floor joist above but with little succes. Sometimes it seem to come from walls other times is seem to come from floor above.
Any sujestion? can mold be found by different lights like ultra-violet or do I watch to much CSI
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