up to 25% packet loss between 11pm-8am

I'll do that this evening when it starts acting up. Do you want the levels without the attenuator in line or with? Here is it now with the -9db attenuator. Usually drops about 3-4db on the upstream at night, without the attenuator it can dip into the high 20's range.

Coax comes from high on a pole outside to the back of the house into a box with a coupler adapter, then down into a hole drilled into the brick and I'm guessing under the house then up through the floor into the room with the cable modem. I'll get a flashlight and see if there are any adapters of any sort in the line under the house. Only the cable modem is connected to the coax from Cox..

Usually drops about 3-4db on the upstream at night

Does the dropping of the upstream relate directly to the time you start to get packet loss? Anything in the logs around the time of the packetloss or do you show any uncorrectables on the downstream channels? So far I am thinking it is weather/temp. related to something outside, maybe your tap.

Yes, but this didn't seem to incur packet loss until a week to two ago, since then it happens like clockwork. I know last year in the summer during the warmest hours, about 2pm to 3/4pm the cable modem would lose signal every day for a half hour up to 2 hours. Since then it seems they've adjusted something or installed new equipment because the signal has been stronger "higher in the +db range". 

Off and on since then half the downstream channels would never lock on. Usually within a week or 2 of this the service would go out around 12am-2am and be down till morning. That cycle repeated at least 4 times, half the downstream channels would go out, within a couple weeks service would go down, then a repeat of the same over and over.

I'll check the logs tonight again, I've tinkered around and reset it at least twice today. From the box on the back of the house where the ground wire is, the coax from there to the modem appears to be a single coax straight to the cable modem.

I suppose it's not actually packet loss so much as it is packets that are beyond a reasonable latency. So far this evening "9:30pm currently" it's working normally with one or two spikes up to 100ms if I keep rerunning the test. In the daytime I don't see any latency spikes and it's near completely smooth as such.

A few more hours and this chart will turn into a disaster. If I were to try playing an online game it would become unplayable as the character movements become rubber-band like, snapping me back to where I was prior and actions I perform not getting to the game server soon enough so they are ignored. I'll post some of those results once that time is here.

I was correct in my first assessment, they were dropped, not late packets. I was asleep during what's the usual worst time, I'll see if I can get more data tomorrow.

Can you share the Pingplotter data?