Connection Problems

hello,

i just built a new system with the help of a friend, and everything is running smoothly except for one thing: the connection.

now, i have a LAN cable plugged in to a wireless router which is plugged into a modem. here's everything i've done...

1) plugged LAN into the router. internet was barely there. it would fuzz in and out, but mostly out.
2) plugged LAN directly into the modem. same problem. reset the modem, and things then worked great
3) plugged back into the router, and reset the modem again. things seemed to be working great...

but then stuff stated going odd
- startup programs get about 90%, then fail to connect in certain parts (i.e. thunderbird will load IMAP to gmail, but not POP3 to hotmail; rainlendar will load my gmail calendar, but not the weather, etc)
- chrome, ff, and ie are all timing out when i try to visit a page, but then load it at blazing speeds when i hit reload (sometimes take a few times to reload)
- and through all this i can play TF2 without so much as a hiccupp [knock on wood]

all through this, i've tried a wireless card, but gotten the same results. and another bit of information is that tyhe ethernet cable i'm using is about 10 years old. but i'm not sure if that makes a

i'm freelancing from home, and a flakey connection is the last thing i want.

any help? and thanks in advance
troy