I've had the same problem for over a year or more even. Comes and goes, usually "corrects" itself by the time a truck rolls around and the levels are always "great" Mean Time Between Failures is 25 Days.
I just cancel and reschedule when it happens.
A couple years ago one tech found some "noise" on the upload side and was to turn it into "plant maintenance" but not sure anything came of that.
I do know that my upload pseed is currently about 1 Mbps slower than normal and my Symbol rate dropped to 2.560 Msym/sec which is a precursor to the full on issue you are having.
Channel ID 222 221 223 224
Frequency 597000000 Hz 591000000 Hz 603000000 Hz 609000000 Hz
Signal to Noise Ratio 37 dB 36 dB 37 dB 37 dB
Downstream Modulation QAM256 QAM256 QAM256 QAM256
Power Level 4 dBmV 5 dBmV 5 dBmV 5 dBmV
Bonding Channel Value
Channel ID 1
Frequency 29500000 Hz
Ranging Service ID 4328
Symbol Rate 2.560 Msym/sec
Power Level 43 dBmV
Upstream Modulation  QPSK
Onsite Computer Support in Memphis