Cogent is one of my network’s upstreams.
I’m a bit baffled by the assertion that the route is flapping. My router’s view of the path to the IP you listed is:
174 3356 13649
Origin IGP, metric 0, localpref 90, valid, external
Community: 174:21000 174:22013 6082:101 6082:9996
Last update: Fri Dec 15 23:42:37 2017
That route appears constant since December 15th.
ViaWest is LetsEncrypt’s direct upstream (AS13649). The Cogent path is presently Cogent -> Level3 -> ViaWest.
Alternatively, my view to the same target via Hurricane Electric is:
6939 209 13649
Origin IGP, metric 0, localpref 110, valid, internal, best
Last update: Sat Dec 16 00:03:12 2017
That’s Hurricane Electric -> CenturyLink/Qwest -> ViaWest
That path also is not updated since a couple of days.