Problem with my Wireless Expander

I have a linksys wireless router connected to desktop & a laptop w/internal wireless card, everything works fine but when I take laptop downstairs to back of house I get no signal. Bought a linksys high gain antenna still no signal. Bought a linksys range expander and now I get excellent signal strength but connection drops after a few min in favor of neighbors low strength signal. This keeps happening as long as I am in range of the wireless expander or untill I unplug it….how can I fix this & why does low sig strength from my neighbor overide my excellent internal connection?