Intermittent VERY slow upload speeds, download fine

My service has been working fine for almost a year since moving here, but for the past few days I've had intermittent issues with my upload speed. I should be getting around 5 mbps but I'll often have periods during which it drops to 1 mbps or even as low as 0.06 mpbs. These periods can last anywhere from a few minutes to a few hours.

I've tried taking my router out of the equation and plugging my computer straight into my modem (Netgear CM500) and nothing changes. Also found a lot of "critical" errors in my modem's event logs such as "UCD invalid or channel unusable" and "Started Unicast Maintenance Ranging - No Response received - T3 time-out", though I don't know if they are related and their timing doesn't necessarily coincide with the upload speed issues.

I'm currently experiencing the issue at this moment (getting 1-2 mbps) and here is what my modem's cable status shows for upstream: