Had a couple of weeks struggling with various login difficuties - including my router (Netgear 834 GT) declining to let me log into its homepage. Only time I could get into the homepage was just after a powerup.
Finally cured by:
- saving my settings.
- uploading the latest firmware from the OEM's site.
- done a factory reset using the recessed button at the back.
- reloading my settings.
- Carefully checked that all of my MAC settings (in the LAN IP Setup Table) were correct.
I went through the process twice before the router settled down. I can only think that either a previous firmware upload had not gone through properly, or that my shuffling around computers in the house (ie, moving MAC allocations amongst names) might have scrambled things.
Hope this is useful.