Just because you can stream with another service doesn't mean your modem is good. I have seen this issue a lot. I set my Neighbor up a month ago with a TV Online+ he had a DOCSIS 2.0 modem he can stream Netflix no problem right? Wrong he had constant buffering totally un-watchable We have the same internet my house was fine so we started talking he had not upgraded anything in a while so he called our local ISP talked to them they sent out a new DOCSIS 3.0 modem and we plugged it in and started the IPTV up worked like a champ. There is a huge difference in speeds on the modems and it sucks that more ISP don't just automatically upgrade you but only wait until you complain.
The other thing you can do is try a VPN service see if your packets are getting throttled that is really the only other thing it can be.