Traffic shaping is broken on 50/10 profile, or I have a line issue

Back when I first upgraded to the 50/10 plan, I was getting a nice consistent, well-shaped 50 Mbps.

I noticed today that a transfer was starting out very sluggishly, but slowly ramped up and at one point was well over 50 Mbps.

Plotting the IO graph in Wireshark shows a very obvious sawtooth pattern:

When I first got the 50/10 profile, it would ramp right up to 50 Mbps quickly and was a flat line (i.e. well-shaped and stayed at 50 Mbps). This packet loss is interfering with VPN, remote VNC sessions, streaming, etc.

When I try to run shaperprobe, it gives me the "Measurement aborted due to high packet loss rate." message (I've tried 3-4 times, always has the same issue):

Estimating capacity:
Upstream: 11579 Kbps.
Downstream: 186281 Kbps.

The measurement will last for about 2.5 minutes. Please wait.

Checking for traffic shapers:

Upstream: No shaper detected.
Median received rate: 11349 Kbps.

The downstream power is a bit on the high end and I've lost sync a couple of times in the last few days, so perhaps it's a line issue, but I'm not sure why it would cause an obvious sawtooth pattern that's indicative of traffic policing. Anyway, here are the line stats (note: I normally have 3 US channels, but I have been losing the 3rd off and on the last few days):

down_channels: 8
down_power: 12.3 12.0 11.7 11.3 11.0 11.6 11.2 11.0
down_snr: 36.2 36.5 36.2 36.2 35.9 35.7 35.5 35.2
up_channels: 2
up_power: 41.0 NaN 41.0 NaN