Router works on and off

I tried to set up my sisters older computer on a wireless network with a new gateway modem from Verizon DSL, installation of card and software went fine, but it only sometimes recognizes the router and when it does it gets a great signal strength, then it wont work at all next time you try to log on. It is an older computer but I figured if it logs on sometimes it can handle the card just fine. If any one cant point me in the direction of wireless network setup directions I would greatly appreciate it. Verizon syas they cant help with the router end of the modem because they arent trained and gateway tries to send you back to verizon once you tell them who the carrier is. The computer in question is running XP sp2.

are you using the network card's utility or do you have xp's zero wireless configuration?? I learned that you can't have both running at the same time, so choose which one you want to use.

Have you tried setting a static ip address for the older computer on the router's device list??

Are you connecting with 802.11b or 802.11g??

I have used the IP address, given by the router manufacturer, and I am using 802.11g. Although both softwares maybe trying to connect like you said. I seem to have sucess when using the cards software, how would I disable xp in this instance, or should I just uninstall the cards software and let xp try to connect?
Thanks