Bart,
I confirm that I've also experienced the same symptoms on Comcast; but
the protocol number is 4 (IPENCAP). I suspected it for weeks and
determined it was occurring last week. I'm unable to tell with certainty
if it's dropped packets due to Comcast, or simply packets lost in transit.
It's not very wise to drop random encapsulated packets; because in most
cases, it would result in more traffic (e.g. a TCP encapsulated packet).
If you still wish to test this further, Ostinato is a packet
crafter/generator.
https://code.google.com/p/ostinato/
An easier method would be to setup a 44tunnel and directly ping another
user's node that happens to be on Comcast (my GW - 44.60.44.1).
-KB3VWG