TX/RX rates drops after 5-10min using Nstreme?

I have P2P 20km link TX/RX rates are 270/300 or 240/214 I can achive speed abut 110-120 mbit but
this last only for 5-10 min sadenly TX/RX drop to 104mbit or 78mbit speed is about 50-65mbit
but if I disable/re-enable some of wirless inerface or reboot one RB TX/RX rates are again 270/300 or 240/214
after 5-10min again TX/RX rates drop
I try difrent version but the problem still presist try configuret data rates
on HT Supported MCS select only MSC12,13,14,15 how can than speed drop to 78mbits
I dont linke to use nv2 because link act very strange and speeds are about 40-80mbit
I have some links with rocket M sory for this but I can achive speed one way about 80-90mbit but thay are rock solid
on this link using RB800 with R52Hn signals are about 62-66 MT ver5.5
pls for any advice or help tnx.