B24 low throughput until frequency changed and then goes bad again

We have a 2.0 mile B24 link that has an RSSI of -68. The PHY throughput is 300/150 and MAC is 72/90. We plan to tune the link to try to optimize this to higher levels but the throughput is sufficient at the moment. The spectrum analyzer shows the spectrum is clear. Frequencies are currently set to 1x40 FD Freq 1-24155 and Freq 2-24090 but we changed them a little bit either way to bring the throughput back.

Very often the PHY and MAC throughput will drop to 5/3 Mbps. We seem to be able to bring the throughput back by merely changing the frequency a little bit one way or the other. But remember the entire spectrum is clear. The bandwidth will come back for a little while sometimes a bit longer but not that long. Then it will drop to 5/3 Mbps again. We have to then move the frequency again to get it back.

We’re on firmware

Any thoughts would be appreciated.
Thank you,

Is there a reason why you’re using FD mode over the regular 1x or 2x modes? Also would be good to see the combined view on the spectrum graph.

Please keep in mind that 2 miles is kind of pushing the B24 to its limit.