Why is my Internet speed slower than usual?

A few days ago my speed was normal(3mb/s), now it is only giving me 1.9mb/s to 2.9mb/s. I have AT&T as my Internet company. I have tried different things such as resetting the modem and changing the phone line on the modem to ha different Jack. I have a 2wire modem. What annoys me the most about this problem is when I'm playing online on my PS3 and someone is using the Internet in the computer the signal on my game(e.g Black Ops) only has one bar which causes a lot of lag on the games. What can I do to make the speed go at it's original speed.