I found that when I had PhyR enabled, it seemed to keep the SNRM low when it did drop.
A channel change may help if you are getting interference from other users.
Download inSSIDer http://www.metageek.net/products/inssider/
- click on Networks and it will show any other users around you and on which channel they are using.
Optimum channels are 1, 6 and 11 as they can't be overlapped as they can by a stronger signal strength user on an adjacent channel.
You should ideally be looking for a channel that will give you a signal strength (RSSI) of around 50 or less with no other near users of the approx. same strength on that channel or adjacent.
If there are any other users on any of the optimum channels that will give you that RSSI value and any other users on the same channel or adjacent where their RSSI >=76 then you can safely use those.
If having found the best channel for you and you still experience these SNRM drops with PhyR disabled, then you should contact your ISP as there could be a line fault - but try changing the microfilter first if you use them.