How to test DSL modem?

Lights will tell a lot, what modem and what lights are shown?

Stratmando

What happen is 5th light is flushing and all of a sadden becomes dark and the stream freezes. When I connect other computer to this modem I have the same problem. The speed is 1200-1400kBs

If modem from ISP, contact their support

If not, check the website of the modem for any troubleshooting or FAQ sections

What Modem do you have, and what does the 5th light mean?
Maybe try another cable if available.
You could try tracert 4.2.2.1 and see if it shows IP of Router if unknown.

Stratmando
This is Siemens SpeedStream 4100. The 5th light is Activity.

Thanks.

Your ISP can do a line test.