Issues with wireless connections

Hi there. I'm a 4 year Mac user, but this is first time I've had this problem. I was wondering if a more experienced user could offer some advice.

I have a 2.4 GHz Linksys router, and I've been having problems connecting my iBook G4 to the internet. There have been issues with our internet provider (Comcast) before, but after a while the problems were dealing with the router. The connection has been slow at best, and it only works about half of the time. Restarting the modem and the router doesn't seem to work for me, and that's the only advice I've been able to find so far. The computer recognizes the network, but every time I try Network Diagnostics it won't connect to the PPPoE server. Is there any way to help connect the computer?

Thanks in advance!