This machine is designed as you were told to be used at full throttle. From experience, if it does this at high revs, I'd be worried about it. The only machine I ever used that vibrated at high engine speeds was a Shindaiwa T260 I think, that had bad bearings in the gearbox.

Thanks crazyhorse, but i understand that at low rpm the cluth is not fully disingaged hence the reason for the vibration. Since the older models don't have this problem, i think Stihl is going backwards in their technology. At some point in time the user has to cut especially lawn grass at a low rpm.

But why use an FS250 for lawn care? I mean, wouldnt an FS80 be better for that?

It is designed to run at full throttle. At low rpm, the clutch may slip and overheat and low frequency vibrations from the head will be more pronounced

I bought the FS250 in order to cut the shrubs in the flower bed down to the ground, like a good brushcutter will. They grow quickly and return stronger than ever. It also does a nice job around the flower beds and trees.
If weed whacking was all that was needed, a smaller machine would do nicely.

I have a Stihl FS81 that is vibrating at any RPM. It will briefly stop at high RPM but as soon as I try to cut, it will start vibrating again. Is this a clutch or bearing problem?

Whether it is the cause of vibration for either machines in question, the wrong grease in the shaft or to much grease can also cause vibrations.

Over greasing the gear box will push the excess up the shaft. The drive shaft should only have a film of grease on it and it should be Stihls shaft grease or other that is designed not to migrate when heated.