What are your line stats over a period of time, do they fluctuate a lot, are you getting frequent disconnects or reboots? An attenuator as sbrook
will tell you may or may not help your particular situation. Theres no point in dropping the RF levels down making your modem work harder to talk to the CMTS just to satisfy a spec range.
I suggest you use the forum search and look up the attenuator threads of which I'm sure there are one or two that clairify this.