First try setting wifi to never turn off. Settings> wireless & networks> wifi settings> press menu button> advanced> wifi sleep policy> never
Then try removing the router from remembered networks, reboot and re add.
Also what kind of encryption are you using? Checking the router for firmware updates is a good idea too. Wouldn't hurt to power cycle the router as well (turn it off and on).
A way to reduce the variables is to see if it happens on other wifi networks too.
Sent from my Android device.