I was running that one for quite a while.. seemingly without issue.. until a while ago. Its possible its that specific modem. (i think something went flakey on the latest firmware)
One thing you could try, is go and take it into the closest rogers store, and swap it for the new GIGABIT modem. (just make sure you get one with one or two black dot stickers back where the power plugs in)
I had no stability issues once swaping. I am on the same plan, and shouldnt be any cost difference.
Only other thing quite possibly be, is maybe there is being a signal drop possibly at different times. You would really need to check the signals at a GOOD time, as well as trying durring one of the bad times. (though, if you can not log into the modem durring the bad times, thats more likely bad modem issue)