I've been going crazy trying to figure this out too. I only seem to have this problem in CRPG. I'll be running around enjoying my 16-20 ping, then I suddenly jump to 500 for 10-30 seconds.
I had something really strange happen the other day too. This has only happened once though. After I hit someone or someone would hit me, the hit would register a second or two after it actually connects. So I would swing, my animation would finish and then a split second later the guy would be hit. It wasn't FPS lag, as my swings and movement were responding correctly. It was so odd.