@deravian 

It is extremely strange to see large ping spikes to a router on a ethernet connection. The only way this can really happen is the router receives the request and is busy doing something else so it holds it in a buffer and responds when it is less busy. This is not uncommon if you were say passing large amounts of data or running vpn on the router which uses lots of cpu. If you are just running ping it should not have much affect.

I am going to suspect dd-wrt. It is very difficult to get a stable release they seem to constantly be adding junk. I would try another image if you can find one. You could also load the factory firmware back and see if it makes a difference. I am going to bet you see cpu spikes when this is happening, maybe it will tell you what process is running.

I don't know if they fixed it but dd-wrt did not support many of the cpu bypass features that lets you run very fast internet connections. It would top out in the 200-250mbps on many routers. Kroger feedback survey... Even if it supports it you have to be very careful what features you use in dd-wrt because anything that needs the traffic to pass the cpu will need to disable that feature. Even simple stuff like traffic monitoring can make it so you can not use the bypass feature.

Note this is also true with factory images you have to be very careful what features you use. Pretty much this means you can not use consumer routers on high speed internet connections to do much more than simple NAT.