If this deterioration has come out of the blue, my guess would be that there's work going on somewhere on your circuit and some of the traffic has been shunted onto your UBR, line card or whatever, interesting how your ping still seems pretty good though.
That or your just suffering from typical over utilisation on your downstream, my speed tests look just like yours (and gaming is frustrating) and I know mine are down to there being too many heavy down-loaders and not enough capacity which is due to be rectified sometime this year.
I'm currently getting a rebate of 15 quid a month for the privilege though