Could you please proofread this short letter

[attitude2010]

Dear Sir

The problem consist on the signal strength which rarely reaches the 3G signal (the blue light in the modem device) so the Internet connection speed is too slow and it takes me several minutes to navigate on a regular website and that if the signal doesn’t fail, which is another problem.

I’ve been facing 2 problems at total that make the service almost useless. I checked the 3g coverage and the city where I live is on the list.

Please do something about it; otherwise I would have to cancel my subscription.

[susiedqq]

One problem is that the signal strength rarely reaches the 3G signal (the blue light in the modem device) so the Internet connection speed is too slow and it takes me several minutes to navigate on a regular website. Also, the signal often fails, which is another problem.

I’ve been dealing with these two problems and I consider the service almost useless. I have checked the 3g coverage and the city where I live is on the list.

Something must be done to correct these problems; otherwise I must cancel my subscription.

[Slow_Learner]

I think some revises may help your letter look more professional. Although susiedqq's revision had helped your letter, I think that by adding an opening statement would serve your letter better. Such as:
Dear Sir [Sale Manager or Consumer Service]

I am writing to inform you about some problems that I have with your device [service].One problem is that the signal strength rarely reaches the 3G signal (the blue light in the modem device) so the Internet connection speed is too slow; it takes me several minutes to navigate on a regular website. In addition, the connection is often failed, which exaggerates my disappointment.

Although my city is in the list of 3G coverage, I’ve been dealing with these two problems for [two months or whatever you want to add here] without any real improvement, thus I consider the service almost useless.

As one of your customers, I would like these problems to be solve quickly, otherwise I must cancel my subscription.

I'm not sure that my revise would help your letter, but I did my best to make it look more polite and reasonable. Good luck.