I've also been having what seems to be similar issues (several minutes of extreme slowdowns / disconnects). For quite a while it only happened between 7 and 9 am, but over the past couple weeks it seems to be happening at other times throughout the day.
I've used both the WOW speed test and Speedtest.net during some of these issues. Sometimes both DL/UL speed are affected and sometimes just UL speed. I have 30/3 and during the "issue" a short time ago, the WOW speed test showed 14.89/0.12.
The smoke ping I've been running for a while also shows the issues:
This is becoming more and more frustrating. I've had WOW for a very long time and have always told others how compared to my experiences with Insight and TW, their internet service has been rock solid. This is certainly changing my opinion.