...if the issue is on my PC? ...Try on another computer on same network, possibly mobile device if Movie provider supports it. Make sure you select same quality options
or my internet provider?Run speedtest http://www.speedtest.net and check speed. For streaming SD you should have a 4Mbps ADSL line atleast
or the Movie provider?If it is a big provider you can probably rule this out but you can test on another device on another network connection
what problem are you experiencing during watching movies ? please describe more.