I posted yesterday regarding the same issue. With some help, I determined it was a duplexing issue. We have three switches in the mix, none of them are managed so it is hard to say exactly what is happening on the switch's end. LED readouts are not always honest with you.
I took the advice i received yesterday and set both ethernet interfaces to half duplex and I am seeing better performance then I was but I am still seeing collisions and dropped packets. I see a lot of spikes even when we are not experiencing any kind of a load.
Again the Router is a Cisco 3640
Three ethernet interfaces, only two being used, the other I have shut. Class C split up into two subnets. Each ether interface routing a single subnet.
Everything looks fine on the serial side.
It could still be a duplexing issue.
Any help would be greatly appreciated.