I need help with my internet..?

I got internet at my house in September and we have three computers, two of them are connected to the internet via wireless USB adapters, mine is one of those two and when we first got the internet it would lose connection every two or three minutes but my computer would STILL say that I was connected to the router, but my web browser would say things like "could not connect to server" or "timed out" and eventually someone fixed that problem but it just recently started happening again, but only to my computer, even though it says that my signal strength is excellent. Now as you can imagine it can get pretty irritating having three minutes to do something before you have to wait a minute for it to connect again, and I can no longer use any of my messengers because I am constantly losing connection. I'm pretty computer literate and can handle most instructions, so if anyone can help I would really appreciate it.