Compass Measurements: Spikes and Offsets

Hi all,

We are using Copter 4.0.7 with two external compasses (MMC3416). The two compasses are located right next to each other. From time to time a compass shows weird spikes or a big offset in magnitude compared to the other compass. Please see some plots below.

Has anyone seen something similar and knows a solution?

Thanks you for any hints.