Short answer: no. It is not possible.

You cannot control the throughput of the modem or your DSL connection. I am curious about how you are determining your speed? Is it just that websites start loading slower? Or are you running an actual throughput test with no other software accessing the Internet?

In any case, there are a few things you can do to get the most out of your Internet connection.

First, don’t use the USB cable. Get a patch cable and connect the modem to your computer’s network card. Best to get a router as you said. I just did a test on my own modem. I get close to 7 Mbps (actual throughput) with a direct connection to my PC’s network card or with a router. However, when using USB, that drops down to about 4.5 Mbps. That’s a huge loss just for using a USB cable. Since your connection is only a fraction the speed of mine, I would guess that makes a big difference for you.

Second, be aware that if you have any software that does automatic updates, or if you have any malware (Trojans, spyware, etc.) that is downloading/uploading garbage, this can account for getting good throughput right after reconnecting the modem and then seeing a drop-off after a few seconds/minutes. It isn’t that your connection became slower; you just are transferring other data which is using up your bandwidth.

Third, you will never get the full connection speed that your ISP advertises. There is overhead, signal degradation due to distance, old wiring, etc. If you are getting 75-85% of the advertised speed in actual throughput, that’s pretty good. Especially for a low speed like yours. I get about 90% of my advertised speed in throughput on a good day.

Try testing your actual speed against several servers:
http://www.speakeasy.net/speedtest/

Hope all this helps.