Volume fluctuation

I replaced my stock kit with new speakers (Focal PF 165), stereo (Kenwood DMX7706S) , 5 channel amplifier (Pioneer D9705) and two 8 inch subs (sundown E8 V5) on a 2016 Honda civic LX. Since last couple of months, I have been facing an issue where the volume on my speakers (both rear and front, left and right) automatically goes down and then comes back up again. The power on the sub woofer is consistent. Think of it like you are listening to music on you phone and a you get a notification. There is no speed control feature on my stereo.

This happens immaterial of whether the car is moving or idle. When I just start the car, the volume is fine. It stays there for about a minute or 2 before the fluctuation starts

I checked the voltage on my amp and my battery and both are > 13.5 consistently. I even charged the battery just to be sure (not sure if this matters)

I turned off the amp completely, reset the stereo but nothing seems to be working. Before I start testing the stereo with only the speaker (by passing the amp), I wanted to check with this forum and see if someone has had a similar experience and would have a better idea of what is going on.

Thanks in advance.

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EDIT: So it turns out one of the speakers (left door driver side) had a loose connection. Fixing that fixed the volume issues. Although I am not sure why there was volume fluctuation in all the speakers.