So we decided to call the ISP to see if they were seeing any issues on their end. Perhaps a high amount of jitter or something. Well they said that they were seeing CRC mismatch errors on their end, and it was most likely caused by a speed/duplex mismatch. They recommended hard-coding 100Mbps Full Duplex on our interface connected to them.
Looking at the GUI for my Fortigate 60C I didn't see an option to set duplex or speed, so I had to drop to the CLI and run the following:
forti60c$ config system interfaceOther options you can set on physical interfaces are 100full, 100half, 10full, 10half and auto. It appears that out of the box, the interfaces are set to auto.
forti60c(Interface)$ edit wan2
forti60c(wan2)$ set speed 100full
If this post helped you, let me know in the comments! Also, if you have any tips for a Fortigate n00b like myself, hook a brother up in the comments as well!