Plusnet ADSL Connection and Sky Boxsets onDemand

[#1]

Hi,
I moved to Plusnet in May and everything was working fine with my Sky Boxsets on-demand.  I recently switched to a new router as the router supplied by Plusnet was not up to scratch.
Since this switch I have started to encounter issues with downloading Sky Boxsets, but the problem is very random. Sometimes they work fine. Sometimes I have to download a random episode of a box set to then download what I actually want to watch! All the other Sky services work fine, and any downloads from iplayer, All4, etc. via Sky work fine!
Now Sky have tried to blame Plusnet, but as I have told them my internet works fine, I get a consistent speed of around 9Mbps (above the 2mbps Sky recommend).
Is it possibly a setting on my router that is causing the issues? My router is a TP-Link TD-W8970.
Any suggestions?
Thanks in Advance

[#2]

MTU settings perhaps?

[#3]

A slight inconvenience with the 8970v1 firmware is that it does not allow an MTU value greater than 1480 to be entered. TP-Link fixed that issue for a similar model (the 8980) about a week after the last 8970v1 firmware was released, but TP-Link have not released any new firmware for the 8970v1 for over a year now. That doesn't apply to the 8970v3 model.