So, did the AirPort Extreme work OK with the same modem before, or was there a modem change recently?
Unfortunately, there are a number of posts about compatibility / speed issues with the Motorola modems. In addition to the SB6813, the SB6141 appears in a number of posts. Personally, I had issues with the SB6141 when I needed a new modem about a year ago, and despite trying several different samples, the issue was not resolved until I tried another brand of modem that the ISP recommended.
AirPort Utility in OS X (10.10.2) no longer has any type of manual settings for the WAN port on the AirPort Extreme, so there is not a lot that you can do.
I would first try resetting the AirPort back to factory default settings and then set it up again to see if that might solve the issue.
Otherwise, you could try inserting an Ethernet switch between the modem and AirPort Extreme. The switch might "talk" to the modem better is the thinking here, which has helped some users with this issue.
Another thing that you could try is a crossover Ethernet cable between the modem and AirPort Extreme,