Hi,
As a thought, it sounds like an overheating issue.... I would position the router in vertical orientation.
If its out of warranty already, you might open the unit up and check for a heat sink issue on the wireless chip, or possibly see about installing a heat sink on the wireless chip. I have never opened mine up so I am not even sure if the wireless chip has a heat sink installed...
You might also see about swapping out the power wart for your router....
It may well be that the unit is simply defective.