I have a wireless bridge between two same RB911G - both on v6.21

TX rate is alwasy between 270-300Mbps. RX rate always jumps between 6-300.

Why is that? Wisibility is 100% good. I made few pics. Please help me get this stable.

http://imgur.com/a/iKkHP

EDIT: Forgot to add. TX/RX CCQ: 98/100%

What throughput speed do you get?
does throughput speeds fluctuate ?

I have this Routerboard connected to simmetric Internet 20/20Mb so I can't test it deeply. But now I ran /Tools/Bandwidth Test and I get TX/RX Current 69.4 Mbps/74.7 Mbps

While testing radio RX was at 270, after test it started again so this is dynamic and based on current bandwidth, am I right?

yes you are correct that thay dynamically adjust