somewhat similar things happened to me, speed halved and ping time erratic as well.
Turns out one of the capacitor in my router was blown, thing just soldiered on, don't know if that's good engineering or not.
I only realised what went on by plugging in non-supported 3rd party modem...
I remember you had to fiddle with vlan settings, not too crazy, just need to find one that lets you do so.
There's only so much help desk can do, with what they have (nothing really?).
Umm, did you open your router to find this out? Doesn't that goes against the warranty of the router????