Hi, may i suggest this could be your net provider throttling the speeds i know from experience of uk providers many of them such as virgin throttle speeds the only one i know thay dont but they do average your speed out at 12am are sky but you still keep a good stable connection, other than that you havent done a thing wrong, but really
Without having a solid connection theres not alot can be done other than switching provider, may i suggest trying game on a friend or familys pc who are using a different provider? To see if you get lag spike then, it also may be graphical issues but the high ping been consiquental, try running game in low graphics to be sure its not graphical, i know if i run myne on max in mp i get a slight stutter and i have a gtx465 overclocked, so i drop it down a notch and problem solved, i noticed alot of people set graphics same as SP but MP graphics have more to load as they are not buffered the same as SP so give it a try dropoibg down the in game graphics and also check your video drivers are updated doesnt usualy fix much but hey ho you never know, im not saying this is your issue but its wortg a try, il be happy to help with what you come back with