So, I found the problem on my C7. Your mileage may vary. In Security/Advanced you can enable/disable DoS Protection. The default values in this are way too low, and at intermittent times they trip,...

I seem to be having similar behavior on a brand new C7 with a slight variant. Android fine. No PCs. Newer ipads and Macbook pro retina with 802.11ac ok, older ipads and Macbook pro retina without ac...