I have them too. Not tried it but found this. It should be done in your Windows.
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Open the administrative tool Local Security Policy.
Under Local Policies, open Security Options.
In the long list of policies, find Network Security: LAN Manager authentication level...
Just to rule out interference, the xbox or so, do you have like a laptop when you can run a ping -t to your router and leave it running for a while, then kill it and see if there is packet loss?
If there is a sdk5 version for the firmware you downloaded, try it.
I had problems with my rt-n66u and with the previous firmware release. Moved to sdk5 and no more packet drops.