I have seen similar varying ping times on my 50 Mb/s business service with 5 fixed IP addresses as well. This used to be much clearer, but right now looks like:-
You are right the older 50 Mb/s uses an L2TP tunnel to deliver the fixed IP service to the Business Suberhub.
The newer services are yet launched with multiple static IP addresses, just dynamic or single static. These services must be close to launch now, as they have progressed beyond trial and are in a small pilot phase right now. I am in that pilot (which is not under NDA) and have the 200 Mb/s provisioned also with 5 fixed IP addresses. There is a similar method used to deliver the fixed IP addresses, but instead of using an L2TP tunnel they use a GRE tunnel. This terminates in a different location in the VM network. This incidentally means you cannot port IP addresses from the older L2TP to the GRE based service so you will get a new block of addresses when the new services are fully launched.
Connected to the same cable, which is split, I have the older 50 Mb/s service running (above graph), and below you can see the same time period on the 200 MB/s service.
I am not in an upgraded area so 200 Mb/s is the maximum currently available here. This is running on 12 downstream channels and 2 up off a Cisco CMTS
