sounds more like your connection is all over the place
that site is always accurate on mine anyway the best test is normally usenet if you pull 6000 or slightly better a second that around 50mb


have you checked what Power levels your modem is at? sometimes these are the cause of poor speeds/ dropouts etc
in your browser type http://192.168.100.1
In the list click Upstream and check
Power Level
(dBmV)
Then click Downstream
Power Level
(dBmV)
post the readings of these
ideally the upstream power should be 50 or less don't
want it in the mid to high 50's or thaat is trouble
in the downstream ideally we want to be close to zero or even minus
can get away with +3-5 but too high and this would be another trouble zone.
)Can i ask what you mean by trouble if the upstream is more than 50 ?
Mine has been @ 58.21 since installed a few months ago and always get drop outs (normally after the bill has been paid)
Unless it's dropping your signal under -10dB I would not change anything - what's your current signal levels?

Your down levels are fine, your upload is a little low though.
edit - try removing the 12dB inline and see what your power levels are again.
Yes quite, sometime mine burst's upto that on news servers, but settles to around 5900-6100kB/s