I have doubts about your network. Maybe your cat/dog eat LAN cable ?

My recommendations:
a) try speed 2 PC's (only 2 PC's and 1 router/switch !!!) Try to copy 1 big file and many small one's.
b) try to do same thing with all pc's in the network at the same time. repeat several times.
c) try to connect only your DNS and PC with crossover lan cable. download/upload.