What could it be?
Before was old version 3.x and x86 router, after changing to rb750 have next problem.
customer is connected with wire, 100mbps through switch. On interface table no drops etc.
connection is something like that:
customer1—…-RB750g-…-customer3
customer2–switchA-------switchB-----------------switchC------------switchD
etc
problem have customer1, at the same with customer2,customer3etc no RX errors 
Have allready connected another PC, changed cable switch-customer, changed port.
Problem appears to be after rb750 install