Yeah, that power level is way too hot. I was having the exact same problem (anywhere from once a day to once a week the modem would randomly drop offline until I rebooted it) and my power level was also in the +11-12 dBmV range. I bought a 12 dB attenuator from Amazon and it completely eliminated the lock-ups. A 12 dB might be a little too strong for your upstream, so I'd start with something in the 8-10 dB range (like this one
A new modem will not fix this problem, but it could make it either better or worse depending on how sensitive the new one is to a too high power level.