I would put money on it being the initial power draw when the AC/hot tub kick on. Why do I say this? Because of what I see with my portable AC. When it kicks on my UPS kicks in for a quick moment. Yes I have it set very sensitive so it is not really losing power but definitely causing ruckus with the power in my place. It is the same thing with my laser printer as well. So really if someone is noticing a drop/loss in signal right when the AC turns on it is power causing the issue.
A little bit off topic, but I've noticed the exact same thing with my laser printer, Huge power draw on startup to the point where it would put my UPS into overload so I had to move the plug over to the non backed-up side of the UPS.