3750x or any other L3 switches have acceptable forward rate and back plane to handle lots of packets! Yes, lots of. Have you checked your cpu and memory and also inbound and out band traffic on involved interfaces?
you can check memory and cpu with show process cpu sorted and show process memory sorted commands. You can also check average traffic using show interface command or monitor them using SNMP.
BTW, are sure about your cabling ? connectors and ,,,,? I mean, are they totally the same in your test environment?