Belkin Router slows down

[#1]

Hi,
I have a Belkin N+ Wireless Modem Router model No. F5D8635-4 v1
Internet speeds are fine, but over time it slows to a crawl, powering the router off and on restores normal speeds.
Info from the router:-
Downstream 8128kbps, Noise Margin 9.5, Output Power 19.9, Attenuation 28.9
Upstream 448kbps, Noise Margin 23.0, Output Power 12.4, Attenuation 17.5
The router has been upgraded to the latest firmware, this made no difference, the fault shows on 4 different PC's connected by Wi-Fi, no other Wi-Fi connections near the same channel.
Any ideas?

[#2]

Don't use wifi? Wireless Is very unreliable.

[#3]

Are you able to tell whether the gradual slowdown is with the WiFi or the Broadband link?
Does it happen when just one PC is connecting?
Do you happen to know if you have the latest software on your router?

[#4]

It happens with 4 different PCs connected by Wi-Fi
The router firmware has been updated to the latest vesion (made no difference!)
Next time it happens i will connect directly instead of Wi-Fi, but it must be the router... re-booting the router won't affect the speed down the line!

[#5]

Did anyone manage to solve this or is anyone else experiencing the same issue?
I got this router a month agao and am experiencing the same issue. It usually happens around 5-10 minutes after I've switched my PC on and it's connected to the router. Initially internet connection and speed are fine but then the signal from the Belkin just dies completely and I can't even access the router login page. It usually returns to normal again after a few minutes but this can be very frustrating if I happen to be gaming when it happens.
I've update all firmware without any success but if this can be fixed by changing a setting or something I would very much appreciate the advice. Thanks.