Speed drops on Ultimate tier

I've had a problem with the speeds of my Ultimate tier Internet for a couple of months now.

I will get speeds of 300+Mb/s at a max, which is good, but very frequently, sometimes with 10-20 minute intervals, the speeds will drop down to less than 1Mb/s speeds and stay there for several minutes before slowly increasing back up to 100-300Mb/s.

Also, the upload speed should be "up to 30Mb/s", which I've seen prior to the issues I've been experiencing, since that I'm stuck at 5Mb/s speeds up, never any more.

The issues have been reported with support on several occasions, two techs have been out so far, first one removed some old filters from the line from when we had whole-house DVRs, second one found our uplink power somewhat low, so he put in a ticket with the Cox NOC to have that checked out.

The uplink power did increase a bit after that, from ~36dBmv to 42dBmv across the four uplink channels, but now that's back down to around 36-37dBmv again.

I should note that I have an Arris SB6190 modem with 24 downlink channels active, and four uplink active, and it's running firmware 9.1.93N.

What else can I do to get this corrected?