i assume if you're behind a router you have more pc's connected at the same time?
if yes leave only your and check a speed;is normal to have a fluctuating ping and speed if other pc's behind the same router use bandwidth and you don't know about it.
it was asked in post no.14 if other computers from your local network (connected to router) has the same behaviour but you didn't answer .
i made my home network also with a router and when i'm using 80% of my bandwidth the other 2 connected has a high ping and low speeds.
you can use software to manage the router bandwidth like:http://www.desksoft.com/BWMeter.htm
also if your router has QoS options you can shape bandwidth but i don't know exactly how because i don't need it...