If you put in an amp when your signal is fine to begin with, it'll likely only cause you problems. Especially if it is only a one-way amp. A one-way will block the return path of your cable modem and your modem wont stay online. Amplifiers, to a lot of people, seem to be the fix-everything trick, yet I see the Radio Shack-purchased ones cause more problems than good.
It does happen every so often that it is an outside-of-house problem that needs to be looked at...
The best bet is to just phone the cable company (as you did) and get someone to come out, and check the cabling to and in your house.