i am not a computer tech or anything, but i can tell you what i did when i had a similar problem, it may help i don't think it can hurt, i had one laptop that was doing the same thing and my ISP came to the house and could not help,they said my wireless on my computer was bad, so i called up my extended service guy told me this
if it is a linksys router go to there website and support page; then click to go to - Technical Support- page, then on left column click -EasyLink Home Networking Tools ; then click - EasyLink Firmware - and download the update for the router. it fixed the problem for me. there is also a link below that one that may help if you have a different router.
good luck.
GQuilts