Richard,
Are you trying to fly the biggest helis? The bigger the more stable. Also, it concerns me that you can fly/hover in real life, but not in the sim. Seems quite odd. I might try to change the sim multiplier setting from 1.0 to .5 or something like that to see if it made a difference. But it might be even better to call Horizon's tech support and get them on the phone to deal with this directly.
Also, make sure that you have all the updates done, complete calibration, drivers updated - maybe you know all this already, if so, I apologize for repeating this.
I know it can be quite frustrating.
Best of luck
Scott H