Corrections in red; suggested stylistics in blue.

A letter

Dear Sir or Madam: 
I am writing to you to inform you about problems with the internet connection on the block where I live. I have a contract with your company which states that you will provide a 1Mb/s speed connection. However, sometimes the internet speed is lower, which makes working on the Internet impossible. Moreover, sometimes I can’t access the Internet.  Instead, the following message is displayed:  “We have a problem with your DNS service.” is appearing. I discussed these problems with my neighbors, and learned that they are experiencing the same inability to access the Internet and low speeds. According to our contract, your company must provide a quality connection. Thus, I am requesting that you check out your lines and cables. Moreover, I would recommend that you to check out your service computer program.

I assume you will bring your service up to the agreed technical requirements.  However, if these complaints are ignored, your customers living in my area may have no choice other than to seek one of the many other providers offering internet services. 

Sincerely,
Alex Harry