Sounds like clipping to me also... Turn down the ffb a bit (or/and a bit more) til the drop out doesn't happen anymore.
There is one other possible cause... But I have only read about it, but, never experienced it... Apparently , in a batch of fanatec wheels that went out a while back, there was some weak soldiering joints on the far end (from you, if you are driving) of the base. During especially long or sustained ffb action, the joint overheats briefly and the wheel clocks way back til the temperature and voltage recovers (which is pretty quickly). I've read that you can test for this issue by applying minor resistance against the wheel when u turn it on and it is calibrating itself. If the issue is present, the calibration will become "chunky" (like "chunk...chunk...chunk") after about a second or so.
Anyway, both problems are pretty easy to mitigate, and don't take long to fix... Also the fixes are permanent which is good (permanently less ffb in one instance)... Post back what the problem ended up being if u don't mind... I'm curious.