With cable, slow downs during peak hours should have went away as cablecos started to deploy D3 10 -12 years ago to take advantage of multi-frequency bonding. First thing I would do is figure out how many downstream channels the cable company is boding in years area, and then make sure your modem can support at least that many. If there are 24 QAMs being bonded, and your modem is an old model that can only support 4 channel bonding, you will have issues. Also look at the signal levels, if the levels are borderline, that can cause problems as well. Signal levels fluctuate with temperature. If It’s borderline during the day, some frequencies may go out of spec as evening approaches and temperatures change.
Most ISPs over provision service by 10 – 20%. I pay for 940 x 35, which is set to 1200 x 42, and can pretty much hit 940 or higher x 40 no matter the time of day on Charter in an area bonding 31 QAM + 1 OFDM on the downstream and 4 channel bonding on the upstream.