All devices tested so far have been wifi? The phone working better tells me very little, it could be receiving a lower bitrate version of the feed, or the wifi on the phone could be better for whatever reason (even if that seems unlikely). Test a wired connection rule out wifi issue.
If the same in wifi, do your speedtest.net results indicate normal speeds at the time of the issue?
Can you run a ping and traceroute from command prompt to the website you're trying to reach to see if any loss happening there?
1100T, Biostar TA870U3+, 16GB DDR3, 240GB Mushkin Chronos Deluxe, 2x1TB Black, 2x3TB Green, 3GB XFX 7970, Antec 650W, CM690, Dell U2713HM & Samsung 2343BWX