this will be down to line noise or traffic...as you know this time is normally quite busy every one is on the internet .
the thing to do is reboot the router at peak times,
around 9 in the evening then use it normally for a couple of days then reboot again at 9 ish ...if you keep the reboots to about that time the router will
keep a good line sync and should give full access day or night.
i have found this site to be most helpfull .
hope this helps, Dave.