With your signal levels as good as they are, they would have to shift quite far for them to go out of spec. You can monitor them throughout a 24hr period to actually see if they are massively sliding. Chances are that the signal levels aren't the issue.
Connect a PC/Laptop, etc. directly to the modem eliminating the router. Monitor your connection for a day; periodically viewing your levels & logs. If you still have issues, at least you have eliminated the router.
Out of curiosity: record if any of the frequencies are changing with either downstream or upstream.