I’ve downgraded rb433 and rb411 6.0rc1 > 5.21 (mipsbe ), and I started to observe interruption of transmission over wifi ( nv2/mcs4/2ms/40mhz) almost all clients 99/100% ccq, no disconnect but tramsmission stops for 10-30 seconds. first of all
i didn’t thought it could be 5.21 ( nv2 “improvements”) but after downgrade to 5.20 transmission wasn’t interrupted… but new problem arised… 100% cpu usage… on both routerboars. Profile says that “unclassified” eats the cpu.
I’m really shocked. I downgraded down to 5.18 but cpu still 100%! Now I can do nothing, neither upgrade nor downgrade.
what is the solution?
i’ve played with firmware as well.
two separate routerboards hanging out for 5+ years. and the same effect 100% cpu usage can’t be by chance.
the cpu usage problem is related that you did a upgrade to RouterOSv 6.rc1.
Could you provide support output file from v5.21 where those transmission stops happen? Send those files to support@mikrotik.com
Maybe you could provide us remote access to those routers?
1/These reouterboards are in “production” so i won’t be back to 5.21 till You solve problem.
So i can’t send You supout 
2/And i REALLY need to know how to get rid of 100% cpu usage, now?
Im using pppoe. Stations are connected in station-wds mode. I was pinging simultanously few of them
and gap in returned ping was for 10-30 packets,
no disconnect but traffic freezes. Im 100% positive,
because I did double check (switching between 5.21-5.20-5.18) on two separate routers.
Switching back to 5.18 solves problem with gaps but those 100%…
btw. im connected to internet through these routers, so I do really feel
when transmission freezes…
maybe 6.0.rc2 would solve my problem?
Could You please send me or point me to 6.0 rc2?
Regarding packet loss:
What are your “hardware retries” settings on AP and CPE?
About CPU usage:
netinstall should take care of your 100% cpu problem
About CPU go to system resources see if the right CPU Frequency is selected for RB433 its 300Mhz
Sometimes it happens to me when I migrate to another router version
like on 433Ah it should be 680Mhz but after upgrade 5.x->6.x it was on 100Mhz
CPU is correctly set to 300mhz, (unclassified eats the cpu )
HW retries = 7
i don’t think its a settings problem, because prior to 5.21 nothing
bad happening. simply upgrading to 5.21 problem occurs, downgrade
to 5.20 or 5.18, works like a charm to gaps but introduces 100%cpu usage.
Also it doesn’t look like wireless problem neither CCQ nor data rate
fall down, but bytes stop to flow.
we would need support output files from those boards that has those problems with Nv2.
The 100% cpu problem is related to v6.rc1 and after that if you downgrade to RouterOS older than v5.21 the CPU could go up to 100%. To get rid of that you need to reset the configuration and reconfigure the router.
wow man, never use beta versions on production.
The only beta I’ve used was 6.0rc1 and it was working without problems, it is official stable 5.21 that introduced problems.
And Yes 
i’ve got some kind of test zone routerboards, these are connected to my place so i’ve upgraded two routerboard out of few hundreds.
And some time ago I realised that upgrading routerboards with new “stable” official releases is somehow bravery
Tue that. For the first time in many years 5.21/5.20 messed up my boards after I downgraded to earlier version.