Hi,
I use two SXT5 lite to connect to a cable internet on the other side of the road (20 m distance). Everything is running fine for two years now, TX rates are pretty stable, but RX rates are quickly varying between 120 MBit/s and 6 Mbit/s. I supply an animated gif for illustration.
I would be very happy to know the reason an, possible, a tweak to fix this.
Thank you!
Bernd
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6Mbps is the lowest of the so-called "basic rates", and the only basic-rate with default configuration.

Radio switches to this basic rate when initially establishing or renegotiating the radio link; this indicates you're having interferences, SXTs may have moved, a new nearby obstacle, or a new obstacle in the LOS, etc... something that is making your PTP to continually (re)establish/negotiate the link.

Check for those potential issues, if you cannot alleviate them, you'll need to "lock down" the maximum datarate, by manually setting allowed ones on Wireless interface > HT MCS.

Start by enabling only up to MCS 10 (90Mbps) and check if datarates stop flapping.

Dear pukkita,
thank you very much for this explanation. I greatly appreciate your support and will follow the approach you proposed.
One interesting thing: why is only RX affected, not TX? At both devices (the device connected to cable and the device in my home).
Anyway, thanks again!

Because there are two directions while communicating.

Let's suppose this scenario: AP <-> STATION using frequency 5640.

Interference may be local and affecting only the receiving side, (e.g. other antenna nearby STATION transmitting on 5640).

while it could be possible that AP can "hear" STATION when STATION transmits without any interference.

Analogy: You're at a crowded bar top ordering drinks to the waiter. Your friend is sitting on a distant table, you ask him what does he want and while he's answering someone else at the bar top speaks to you; you may be able to "hear" something, but won't be able to understand it as it will be overpowered by the person near you.

Also remember that multicast are sent at 6Mbps if multicast helper is not set to full.

Dear pukkita, dear docmarius,
thank you for your explanations and support. I changed frequency to 40 MHz width 5300/5320 (channel 60-64) as this band is totally free of interferences, there are only two other access points at 5180/5200 and 5220/5240. I also disabled higher data rates as suggested by pukkita, enabled Adaptive Noise Immunity and Multicast Helper = Full at AP (mode bridge) and client (mode station bridge) as suggested by docmarius. It seems to be a bit better, but still drops to 6 MBit/s occur. I will now try to align AP and client to improve CCQ value and stability. For setup shown in GIF I allowed higer data rates again. Meanwhile, the problem with changing channel has been solved by fixing channel list.
Here, an even worse example:
When changing this setup to HT MCS rates lesser or equal 10, drops to 6 MBit/s did not reduce.