Speed tests say 6-7Mbps, but can't stream video

Question

I have a brand new BT Home Hub 3, connected to the main socket, through an i-plate and I am on the Option 3 upto 20Mb broadband package.
When I run a speed test I typically get 6-7Mbps which whilst rubbish compared to the 'upto 20Mbps' I am paying for should be sufficient. However sometimes if I try to watch BBC i-player on maximum resolution through my PS3 it stops and loads every few seconds. I know streaming high resolution video is fairly demanding, but not that demanding, I am not doing anything else at the same time so I think 6-7Mbps should be plenty. This only happens sometimes, but if I am trying to watch a film around 08:00pm on a 'school night' it happens more often than not.
If I drop the resolution it normally settles down, so I guess I am right on the edge of what it can handle, and I have a sort of 'work around'. But I am paying for the top level of broadband BT offer (they haven't put fibre optic in my exchange yet) and have the top level of kit BT supply, and I am not asking for the moon on a stick, just BBC i-player on top resolution.
I am normally connecting the PS3 wirelessly but I always have 100% wireless signal when I check on the PS3, the Hub and PS3 are less than 3m apart, in the same room, with nothing between them and no kit that could be causing interference (except for the Plasma TV near the PS3, but it wouldn't be much use if it wasn't connected to a TV!) and a couple of times I have had the same problem when I have connected them through an ethernet cable to check. I have seen a few bits of guidance saying try a differnt channel, but I think the Hub 3 does this for you automatically.
I think sometimes when you stream video it takes a few seconds for BT to realise that you need more bandwidth and open your pipe up a bit - I am used to the video pausing and buffering a few times in the first minute then settling down - but this can go on for a whole hour.
Please could someone step me through the diagnostics I should do as a first step. I don't know much about routers but I am prepared to go in a play with settings if someone who knows what they are doing will hold my virtual hand.

Answer 1

1. Best Effort Test: -provides background information.
Download  Speed
6.02 Mbps
0 Kbps
7.15 Mbps Max Achievable Speed
Download speedachieved during the test was - 6.02 Mbps For your connection, the acceptable range of speeds is 2 Mbps-7.15 Mbps. IP Profile for your line is - 6.18 Mbps
2. Upstream Test: -provides background information.
Upload Speed
932 Kbps
0 Kbps
832 Kbps Max Achievable Speed
Upload speed achieved during the test was - 932Kbps Additional Information: Upstream Rate IP profile on your line is - 832 Kbps
We were unable to identify any performance problem with your service at this time. It is possible that any problem you are currently, or had previously experienced may have been caused by traffic congestion on the Internet or by the server you were accessing responding slowly. If you continue to encounter a problem with a specific server, please contact the administrator of that server in the first instance.
We have identified that you have additional services enabled on you broadband line, please click continue to run advanced service tests.