I'm not sure what the problem is, but there's a few more troubleshooting steps you can try.
Try resetting your modem + router. Unplug the power from both in any order you'd like. Wait 30 seconds, maybe up to 10 minutes if you don't mind the wait. Plug the modem in first. Be sure that all of the lights, specifically Uplink+downlink or something similar, are on before you plug in your router. Then lastly, of course, plug in your router and wait for the correct lights to come on (likely Internet [which just verifies that something is plugged into that port], and perhaps a power light and one other). Then try to use your computer as you normally would.
Is your computer connected via wired or wireless to your network?
When you are having issues with websites loading, are you able to ping various websites? The next time it happens, open command prompt and type in " ping google.com -t ", or substitute google.com for any other site. Perhaps even ping your router or modem if you are aware of those addresses. If the pings to your router/modem vary wildly (ie: time=1ms time=200ms time=800ms time=50ms time=1ms), you likely have an issue with your local network. Similarly, if those pings are fine and you're getting variation in your pings to the outside world, there might be an issue with something between your house and the outside network.
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