Ok, well some cable modems require you charge them the very first time before you use them. (leave them plugged in over night etc.) If you do not do this the power cell only charges up to the point of first use. In otherwords, if you only allowed it to charge for 2 hours, your only going to gt 2 hours of good performance. let it run down completely, then turn it on (offline) and let it charge until it is at its maximum.
If this is not the problem then I'de definately go with the "slow cable modem during busy hours" theory.
Hope this helps :)