Instant research hit here on the forums:
Your router may cause this if it tries to verify internet connectivity via DNS Probe, as described in Seems to block everything after a few hours - #18 by xelemorf (currently the very last post by xelemorf in there).
I will be off soon for a long weekend, so I won't be able to assist you much longer.
In the meantime, try searching the forums for your router model, or alert a mod or some of the guys from yesterday by @ing them 