periodic signal decrease with FID

I was experiencing ghost peaks in my water blank shots so I decided to do some maintenance. This included a new S/SL liner and septum, removing the first inch or two of the column, FID disassembly and ultrasonic cleaning of the jet, and reassembly. Now I have a different issue. At rest, my signal decreases periodically, then returns to zero, only to decrease again. See picture.

Any ideas what could be causing this new issue?

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