It depends on the reason the gateway was marked down. If it marked it down because of "loss" then perhaps your gateway did stop responding to pings, either because it drops out like you said, or because there is actual loss on the line.
If you watch the gateway status widget on the dashboard as it happens, you should see the loss or latency as it gets marked down. The quality RRD graph should also show you.
You could increase the thresholds for the WAN to be marked down on the gateway settings, and also change the monitor IP address to something besides your gateway (DNS server, google, etc).