Assistance with Vibrations on hard mount cube

It seems i am encountering a high Z vibration on a 1400mm Hexacopter with 22inch props.
I have been trying to figure out solutions to solve, since as its noticable that three minutes into the flight clip0 jumps to 40 and 5 minutes in, increases to 900+
Clip1, 5 minutes in jumps to 32 and clip2 jumps to 300+
At the same time X-Y vibes peak at 30 while Z goes crazy and accelZ is pretty close to max values in many occasions.
Is there some “soft” dampening solution, since frame has been checked over and over for flex and vibrations.
The whole issue causes the HEXA to start rolling (5minutes in) at extreme angles and after about a minute it recovers and starts flight normally again.
Any assistance would be appreciated

(Upon investigation under the Cube, i found that the actual base plate has flex and in the meantime i will switch it with a 2.5mm carbon to verify if this will lower the issue, besides this structural fix, since flex is almost zero for the size, would ins_accel filtering help?)

50 in 5 minutes is insignificant
It reads at 4000 x per second…

Post a high rate FFT log.

Finally after a long while i was able to make some changes on “things” that might of caused vibration and run a small full payload flight to create FFT file.
I will run another test Not with default Bitmask, since for some reasson i dont have Vibe and Clip info, but on my live screen my XY vibes never exceeded 10 and on Z i had only once a peak of 40 which i think i know how i can fix.
You assistance is highly appreciated.
2019-05-28 (624.3 KB)
P.S.: For this test i had the Cube 100% hard mounted with a single 1mm of 3m Clear VHB tape so that i could measure all possible vibrations.

@philip I would highly appreciate your input on this.
I made a number of changes on the frame and here are all three imu FFT results.
Since documentation on FFT is not really detailed, i would like to know what you think.
The cube is mounted with the 6mm foam tape.

I had marginal z vibrations on my new build. Decided to have a go at improving it. I corner mounted the cube on TPU arms. Seems to have knocked down the z vibrations to the same size as x,y. Well inside good levels,image

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