Are the pings working consistently and reliably and at a sensibly low m/s?
Some time ago I had a similar issue, but with erratic pings. I did all the usual stuff with cables out and in, re-sets, turning modem and router off and on, gentle thumping of pc where the cable goes in etc.
Turned out to be wiring where the phone line comes in to the house at the "main" socket. (Only 5 feet from pc). Only part-connecting, and only part of the time. BT man arrived, cut off the bit of cable that was s'posed to connect, moved the connection along a bit. Made sure connection was fine. Job done.
The curtain bumps in to the main socket.
Socket was obviously annoyed by the curtain, so went all erratic.