Alliance HPLC PDA 225nm : Disturbed baseline


I have a question about a recent problem which occur in our lab. 
This occur at the end of a samples set run (about 30 injections), The column has 900 injections. but only on few injections.

Here you have a superposition of 4 blank sample (ACN 28%). The chromatogram type is in black.

I don't know the reason of this problem. If someone can help me on this case ?
Thanks a lot

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