C2960 Stack slow network speed only 450-500mbps - WS-C2960X-48FPS-L

[kkana]

So we just noticed our max throughput on one of our network stacks is only getting around 450mbps when doing speed tests.  When I connected to our other stacks, I get around 950mbps when doing a speed test.  This is the only thing I noticed that is different from the other stacks:

Gi3/0/51 connected 1 a-full a-1000 1000BaseSX SFP
Gi3/0/52 connected trunk a-full a-1000 1000BaseSX SFP

The port 51 is not configured as a trunk port. Would this be the cause of our slow speed?

The running config for those ports doesn’t seem to have any special configuration settings on them.

interface GigabitEthernet3/0/51
!
interface GigabitEthernet3/0/52
!

One of the other stacks which gets closer to the 1000mbps speeds:

Gi1/0/51  connected trunk a-full a-1000 1000BaseSX SFP
Gi1/0/52  connected trunk a-full a-1000 1000BaseSX SFP

!
interface GigabitEthernet1/0/51
switchport mode trunk
!
interface GigabitEthernet1/0/52
switchport mode trunk
!

Any thoughts on why this stack is only getting about 1/2 the throughput speed?

[marce1000]

               - Check port counters on the ones doing bad performance
               - Always useful is to verify the problem against the latest advisory software version for the involved platform(s)
                               https://software.cisco.com/download/home/284795741/type/280805680/release/15.2.7E10

 M.

[kkana]

Port counter Errors:

1/2 Speed Stack:
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize OutDiscards
Gi3/0/51 0 0 0 0 0 0
Gi3/0/52 0 0 0 0 0 282268

Port Single-Col Multi-Col Late-Col Excess-Col Carri-Sen Runts Giants
Gi3/0/51 0 0 0 0 0 0 0
Gi3/0/52 0 0 0 0 0 0 0

**ll Speed stack:
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize OutDiscards
Gi1/0/51 0 0 0 5 0 29286
Gi1/0/52 0 0 0 2 0 0

Port Single-Col Multi-Col Late-Col Excess-Col Carri-Sen Runts Giants
Gi1/0/51 0 0 0 0 0 5 0
Gi1/0/52 0 0 0 0 0 2 0