What wireless modes are you using? Try changing from mixed to single mode G or mixed G and N. If your wireless devices are not using A B or N then you really don't need to broadcast these modes.
There could be some interference from other near by routers or cordless devices.
Any house cordless phones?
What wireless devices are being connected?
If your using PPPoE on the router, I presume your on DSL? I presume your DSL modem is bridge?
Make sure Primary and 2ndary DNS IP addresses are filled in under Setup/Internet/Manual if there not auto filled in.
Sometimes it's not the router however its the DSL modem disconnecting from the ISP for some reason. The routers logs will reflect this and might mention a PPPoE connection is down. Thats signafies the DSL modem lost connection, not the router. Check the logs for this.
Can try changing the Firewall settings from there default to EndPoint Independent for both TCP and UDP.
Also check out Advanced/wireless/WLan partiton, uncheck it and Short GI if they are checked.
Can turn off Protected WiFi. Maybe WISH too to test.
Let us know if any of this helps.