Erratic Rx Rate - fluctuates?

Hi

i have a 5ghz omni which talks to 3 x 5ghz directional APs

the Tx and Rx fluctuate however the Rx seem to much more.

for example Tx can be on 54mbps and Rx on 6mbps (see attached)

then it can go back to Tx 54mbps and Rx 54mps

why would it change so much?

what affect could this be having?

thanks
5ghz_.jpg
5ghz.jpg

Does anyone know?