If that uplink monitor is to the internal interface of your upstream device and you're seeing packet loss then the issue can be tracked via packet capture. Generally, when you see loss on that graph it's because the MX isn't seeing those packets come back. Running a pcap on the MX would show that this is the case. If it's not the case then that's something we'd like to look into on our side.
If you do find that a pcap shows the missing packets like you're seeing in that graph, figuring out if the issue is with the MX or the upstream device would mean running a pcap between the devices. I keep a hub on hand to put between devices while my laptop is in another port so I can run a promiscuous pcap in wireshark. If you see the packets not coming back from the upstream device, their lies the issue. If you see all the packets in this in-between capture but a simultaneous capture on the MX doesn't show them then, again, we'll want to look into this in Meraki Support. Please keep in mind that adding a hub to the mix would reset connections and you might not see loss anymore for a period of time.
From my experience, the reason you start seeing loss after a period of time that can be fixed by a reboot is because of traffic prioritization on the upstream device. When you setup a monitor like that or use the default one to 8.8.8.8, the MX sends a constant stream of ICMP to the IP address. Some devices will see this traffic as unnecessary and after a time will de-prioritize it or just drop it entirely.
During periods where you see steady 1% loss to the upstream device, do you see the same loss in the 8.8.8.8 graph?
Alex S | Cisco Meraki Product Management - Cloud