Change to FTTC & effect on TBB ping graph

[#1]

My change from ADSL to VDSL "fibre broadband" took place last Friday (26th July).
The fibre broadband appears to be working very well; Speed test are OK, pings (from the router) to the BBC are around 11 to 14 milliseconds etc, Streaming is working OK etc
The only issue I have seen is the effect on my Think BroadBand ping graph @ http://www.thinkbroadband.com/ping/share/bd5369f77cfa3aec137e749767fc2833.html
When on ADSL I was using an TP link archer C7 router via a DG834 in "modem only" mode..
On VDSL I am using the same router, with the same configuration now via the Openreach modem.
Last Fridays ping graph is attached (tbb ping 26 july 14.png). The ADSL to VDSL changeover occurring between 3 & 4 PM.
On ADSL the peak and average latency  behaved as expected, only increasing when the modem & router were "busy". Ignore the packet loss before midday, that was on TTB/LONAP issue apparently.
As the TTB pings are responded to by the TP link router & not the Openreach modem I suspected the router.
So last night (10PM to 6 AM) I swapped the TP link router for the PlusNet supplied TG582 FTTC router.
There were no devices  connected to the PN router and the wireless as disabled.
See attached graph: tbb ping 30 july.png
The TBB ping response behavior was the same.
Could it be the Openreach modem causing the latency?
Any advice would be appreciated.
Richard 

[#2]

Are you 100% sure you have the same IP address as on ADSL?

[#3]

Hi thanks for the idea.
I am sure my static IP has not changed. The TBB graphs did & still pick up when my PPP disconnects etc.
I have been experimenting and it appears that when we do more intensive streaming (for example a loveflim movie) or downloading (say a linux DVD iso) the graph ping times reduce and stay reduced for some time. Then eventually the ping response reverts back to the more random timings.
I downloaded a Debian ISO yesterday at about 6PM (it took several minutes)  and then the TBB ping response remained low until about 7.30 PM.
All very odd!
Richard

[#4]

Again, just a random idea, but that might suggest something like peer-to-peer activity which is set to throttle back when the device/machine it's installed on is using the network connection for something else.
Doesn't fit with the different router with nothing connected, though...