Comatose Pixel C

I have had my Pixel C for 10 months and have generally loved it. For the first time since I bought my Pixel C it wouldn't wake up when I opened my folio cover. I had to hold the power button for a count of 15 and then it rebooted. I closed the cover and it went to sleep (the light bar on the back went out). I opened the cover, and nothing. I had to reboot again. Since then it has worked intermittently. I have not added any apps to my Pixel C in at least three weeks, though Google Play does update them regularly. And as I am typing this, I am getting a noticeable lag for the first time ever. I use Lookout and am generally careful in my surfing, so I am at a loss. Anybody else experiencing this?