Who is your provider? There is an issue with the firmware being pushed out to that modem, through Comcast users that have it. Have not heard yet of other providers having problems with that unit. But if I had to make an educated guess. Most likely other providers are also having issues with it.
I have the SB6141 and never have had any issues with it. But hearing more then I like about the 6580 series.
I doubt the modem is the problem but if that is the case then there are adjustments you can make to this modem that can help. The reviews in the following link show how to make these adjustments. They mention turning off flood control etc. may be worth a try.
Why can't they just use the older model SURFboard modem like the one I have. I'm definitely not a fan of combining a modem and router into one. Too many problem with those. The reviews for this modem are mixed, it works fine for most but for others not so much. Most mention that you need to turn off IP flood protection....
When I first hooked it up I had a few disconnects until I disabled IP Flood Detection. As one reviewer already stated it is located under the Firewall page and by default it is enabled. Bad Motorola. It should be disabled by default. It's been running 6 weeks now with no disconnects.