Seems like i spoke too soon, youtube videos buffering a lot now at even 720p !! after restarting the modem it seems to work perfectly fine for some time. speedtest.net shows 6.9 mbps down and 1.70mbps up.
My earlier plan was 4mbps/1mbps, youtube at 1080p used to work fine with little bit initial buffering.
anyone facing the same problem.