Wildy Variable wireless speeds

I have a new wireless N Bebox router/modem. I also have a 2010 Core Duo Mac Mini and a current issue MBP, both running Lion fully patched and a NAS connected by 100mbps ethernet to the router.

I have done a network scan using kismac and established that channel 12 is not in use by any wireless networks in range.

Neither wireless device is more than 4m away from the router.

I am not using any wirless g clients.

My wireless speeds as measured by WiSpy fluctuate wildly between 5 & 130Mbps, meaning I cannot stream any video,let alone any 1080p content, from the NAS to either mac.

I washaving similar probs using the bebox wireless G router and a Billion wireless N router.

Unfortunately, I don't have a non-mac computer I can use to measure the network using another OS.

Any ideas what's going on here?

cheers

Does that wireless router support the 5Ghz Frequency? Also do you have anything around the box that could be causing interference like cordless phones etc?