Intermittent network issues reported

I have a client who is reporting intermittent network slowness on a metro ethernet connection and I'm at a loss as to what the issue may be. They have 2 satellite offices that primarily send traffic into their central office they both sites report slowness at the same time so my guess is whatever is going on is affecting the central office. The ISP says they're not seeing any issues. I don't see any issues. Interface details all look good there are no dropped packets, no fragments, no errors. Wireshark between nodes during testing doesn't show anything that stands out but we haven't been able to test during times the issues are present. There were a couple things I noticed while testing that I don't think would cause an issue but I'm not sure.

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1) we're running Jumbo frames at the central site, it was originally configured for the iSCSI VLAN and due to some issues with our fiber SFPs, we swapped ports between VLANs and thus everything eventually ended up with jumbo frames enabled - and stayed that way. My question with this is, since the metro ethernet connection doesn't have jumbo frames enabled could that cause intermittent speed/latency issues? I assume if there are fragmented packets causing problems it would not be intermittent.

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2) Sort of related to #1, while using iperf to test nodes across the metro ethernet connection TCP traffic on a single connection was limited to 100mbps but running parallel connections would get up to their advertised 300mbps. Any ideas as to why this would be? UDP testing showed significant jitter >1200ms and I was unable to run parallel UDP connections - I presume it was just timing out because the test just never started despite letting it sit for 10+ minutes. Ideas as to why?

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3) I ran an extended iperf (5 minutes) and saw that connections would average around 25mbps but would randomly drop to single digits for short periods of time. I assume this is due to other traffic on the connection but it isn't what I would expect across a dedicated circuit.

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Are there any troubleshooting steps I have overlooked? My next step is to setup PRTG to monitor the switch port metro ethernet is connected to and see if we can capture data from times there are issues.