MAC - internet connection problems

Hello -

My CEO owns a Macintosh Macbook Pro that Apple gave him. It has been plagued with internet connection problems from day one and no one has been able to diagnose and resolve the problem. He is frustrated and desperate.  I have a limited MAC experience myself.

He had no problems with his previous PowerBook. He had internet speed problems from day one. He thought it was AT&T dsl, so he switched to Comcast, but the problem remained. Comcast it's his Mac. Mac said it was Comcast. He has problems connecting. When he is connected, his signal strength indicator fluctuates and, eventually, grays out and he loses the signal. It can happen, alternately, as often as every 30 seconds or so. Same problem with Apple Mail. He keeps getting error messages saying that it can't connect, or it asks him to enter his password. When he does, it keeps asking him for his password. When he is on the internet, if he is viewing a page and click to go to another page, most of the time, that freaking spinning ball has it's way and eventually, he gets a message saying he is not connected to the internet. An Apple technician said that both of the problems are tied into one another. They suggest him backing up his hard drive, erasing and reinstalling all of his software and if the problem persists, bring it in to them. He is almost at the point of buying a pc, which he never liked, but no one he talked to who works on a pc has experienced this problem. Note that his sons have a pc's and have none of the problems he is having. It doesn't matter if he is connected via ethernet cable or wireless. He took it to the Apple store and the problem couldn't be duplicated. He took it to Starbucks and even though he had a strong signal, he couldn't make a connection.

Any MAC experts with a god advice here?