Streaming HD movies via internet

Finally got round to connecting the TV (Sony 40NX713) to internet via ethernet cable. 

Have been investigating streaming HD content:

1. LoveFilm - SD is OK but HD trailers will only run for approx 30-45 secs before pausing to buffer the next bit.

2. Using other HD content eg UStudio is OK, albeit these are relatively small 'clips' rather than large films.

3. Unable to compare QrioCity Video on Demand (without paying!) - trailers appear to be SD only.

My connection is a solid 6Mbps.

Is this normal?

Do you require a higher speed for reliable HD streaming of films or is this a LoveFilm peculiar problem?

Cheers,

Colin