WHY DOES MY INTERNET CONNECTION GET DISCONNECTED ?

[gogetaboy]

I have a 512kbps internet connection, anyway the highest speed is 50kbps, And some times my internet starts to continuously get Disconnected, even after i reconnect it, it will get disconnect. and sometimes the speed drops to 2-4kbps which is so irritating (webpages take minutes to load) I found out only I am having the problem bcoz one of my friend is using the same type of connection and he doesn't experience any problem

Can someone explain why does this happening?
I am using an ADSL 512kbps USB MODEM
I called the service provider, they sent some people to fix, but then the people came and they did some work in the telephone line but still did not work, I had called them about 3 times.