Extremely unstable internet connection

Hello. Looking for some assistance. I'm at my wit's end here.

For the past 3-4 weeks I've had an extremely unstable internet connection. Download speeds seem fine but I get virtually no upload speeds. Any speed test I run shows around 300 down, which is great since I'm on the ultimate plan, but the upload tests almost always time out, or the result is extremely low (<1). 

My modem, the Netgear C6250, uses 12 downstream channels and 4 upstream channels. It only locks one upstream channel even though it's normally supposed to lock all four, and the power level running to that one channel is at a constant 57 dBmV. From research that I've done I know that power level is way too high and it most likely is what is causing my modem to reboot itself, which it does multiple times a day.

I live in an apartment complex so I don't have access to any wiring aside from the coax jacks on my own walls.

I've had two tech visits in the last few days. The first one replaced a bunch of fittings and stuff outside along with the coax jack that my modem is currently plugged into. He also informed me of an outage that had been fixed the previous day. He seemed to have fixed the problem. When he left, my modem was running perfectly, all channels were locked in and my speeds were better than ever. Two hours after he left the problem returned. A second tech came the next day, checked things out, and again, when he left my modem was working perfectly. Again, a couple hours later the problem returned. 

Here is a screenshot of my modem's cable connection at this moment.

I assume the insane number of uncorrectables on my downstream channels are another big issue. I unplug my modem to reboot it a few times a day but it doesn't help at all.

Please help. I don't know what else to do. I can't even watch a simple youtube video without it buffering constantly.