wireless strength varies-no cause?

At home we have a wireless/cabled network. WinXP laptop with wireless card. Dlink Access Point (o 802.11G. Dell XP computer attached to network with Cat 5 to hub. Cable modem connects to hub. Network runs great. Accessing internet with laptop shows speeds at varying rate anywhere from 1Mbps-48Mbps. This is during a 5 minute interval. Up and down the signal strength is. Once in awhile the laptop gets knocked off internet cause of slow speed. We haven’t had this problem previously. Nothing has changed. Do you think there is problem with laptop card – can it overheat? No new wireing or interference causes.Nothings changed. Before i invest in new network card for laptop, any suggestions?