Media Bridge - 5GHz - Linkrate drops down

[Cumulonimbus]

Hi,

i have two AC66 in Media-Bridge Mode. If the Media-Bridge connects to the Access-Point, first Minutes or in the first half hour the Linkrate ist 1170 Mbps, then the Linkrate drops down to 351 Mbps.

Operation Mode: Media bridge Firmware:3.0.0.4.374.35_4 (Merlin build)

Any Idea, whats going on / Solution?