optimum cable has two type of modems

When I subscribe cablevision, I got the small motorola modem and the speed varied from day time to evening time. At evening time I sometimes got 1Mb only. I called and called and got and got visited and the situation did not get changed. Back two month ago, I called again and got a 'Monstor' Cisco Modem from the company and now my speed always 12-15MB any time. I was told that in the large population area, the bandwidth runs out. W/ the big and new modem, it can auto solve this issue.