Amazon Prime jittery HD streaming

[#1]

Since getting fibre installed last Dec (40/20) been getting close to max speeds as close to cabinet. Been great for hd streaming via Amazon Prime. Stream would usually go straight to hd mode. However for last 2/3 weeks been getting jittery hd pic & sound (no buffering though), or much lower quality stream never reaching HD level. via Ethernet & wireless. Stream usually reverts to non hd. Using HH5 & W8.1 Acer E1 572 core i5 6GB ram laptop nearly a year old. this happens all times of day. Speed tests usually come back ok, upwards of 35mb download. Have tried usual recommended fixes of re installing Silverlight etc. Happens on all browsers, I.E. , Chrome & Firefox.
Last night discovered that if I unplug mains adapter & run on battery jittering stops & hd plays smoothly,though can still take some fiddling to get it up to hd stream. Starts jittering again if mains plugged in again ( also tried different wall sockets & extensions). Could it be dodgy adapter? Though machine seems to be ok on mains for everything else, connection doesn't appear loose etc.
This seemed to start after the big Windows update earlier in March. Tried a system restore but had no effect.
Have also tried with BT modem & PN router, still same problem. BBC Iplayer hd streams ok, 720p. Think Amazon do 1080p if connection good enough which it was. Usually plug ltop into tv via hdmi.
Thought it may have been Amazon prob at first but been going on for too long now.
Latest speedtest
http://www.speedtest.net/result/4243008659.png

[#2]

You don't say hat device you are streaming with.
The Fire TV box, which you do not use as it only works on mains, covers this in the manual. On the odd occasion it happens just unplug from mains and restart the box. Fixes it fine - something to do with buffer loads apparently.
If you can say what you are using you will likely get a more definite answer.

[#3]

The info is there...

Quote from: Grezzer36

Using HH5 & W8.1 Acer E1 572 core i5 6GB ram laptop nearly a year old. this happens all times of day. Speed tests usually come back ok, upwards of 35mb download. Have tried usual recommended fixes of re installing Silverlight etc. Happens on all browsers, I.E. , Chrome & Firefox.

Sounds really weird.  A faulty mains adapter could cause interference that affects the modem performance but it's unlikely to cause a problem with the laptop.  Is there anything very different between mains and battery operation in Control Panel|Power Options that could explain it perhaps?  You'll probably need to delve into the advanced settings to find anything.

[#4]

Quote from: pwatson

The info is there... Quote from: Grezzer36 Using HH5 & W8.1 Acer E1 572 core i5 6GB ram laptop nearly a year old. this happens all times of day. Speed tests usually come back ok, upwards of 35mb download. Have tried usual recommended fixes of re installing Silverlight etc. Happens on all browsers, I.E. , Chrome & Firefox. Sounds really weird.  A faulty mains adapter could cause interference that affects the modem performance but it's unlikely to cause a problem with the laptop.  Is there anything very different between mains and battery operation in Control Panel|Power Options that could explain it perhaps?  You'll probably need to delve into the advanced settings to find anything.

Oops, was looking for a streaming device ...
Best bet is to look at Power Options and set to High Power rather than Battery Save or similar, that will make sure the laptop is running full pelt at least.

[#5]

Thanks BruceMcTavish/pwatson. Was wondering if some change had somehow happened in power options but hadn't noticed anything strange. Will follow up on your suggestions & report back later.
One other thing I raised on forum some months ago after fibre install regarding BBC iplayer downloads but nobody seemed to have experienced it. When trying download (not streaming), usually starts off at 2/3 mb  rate. As soon as downloaded amount reaches 12MB, dload freezes for few secs then starts again but dload rate plummets to 700 - 900 kbps & never recovers. This happens on above W8.1 laptop & old XP pc. Also happens whether using HH5 or BT modem & PN router, wifi or wired. Iplayer dloads on Android tablet work fine at proper speed. Don't have problem with any other dloads. ThinkBroadband test dloads come at proper rate for my 40/20 connection. Have tried with firewalls & Anti virus off (Windows Defender) but still same. Still puzzles me, is some setting not quite right on line.

[#6]

Just had quick go at Amazon Prime streaming. Switched power plan to High as suggested (previously Balanced). Using mains power ran HD stream ok no jittering. Still didn't get up to HD as quick as used to but that could be Amazon or network issues. Previously always ran on Balanced with no probs for streaming during 1st 3 months of fibre. Who cares if this fixes it. Makes sense as machine must have to work lot harder for HD streaming. Will see how things go.
Iplayer download issue still same though.
Cheers for advice guys. Forum invaluable as ever.