unstable Internet connection

I've been having unstable Internet connection for about 2 weeks. My Internet keeps dropping for about 10+ times in few hours. I called at&t last week and was advised to get another modem. I got my friend's modem just to try it out. I've been using my friend's modem for 1 week now, still having the same problem. So I called at&t again..representative said I got a defective modem. So I explained to him that this is my 2nd modem already and my friend is been using this modem and it was working fine in my friend's house. If both modems are having the same problem and one modem was working fine at my friend's house I really don't think the problem is a "defective" modem. I asked the representative if they could send out a technician to check it out he said there's a possible charge of $99. I've been calling at&t so many times in the pass 2 weeks and I could not get helped, I really don't think that I should pay for the tech service if they could not help me over the phone. The only advice I got over the phone was changing the modem..which I did...still not working! ! Instead of trying other way to help me or transferring me to somebody else that might b able to help, the representative just kept trying to sell me the u verse service. I am so upset at this problem..does any one have the same connection problem???