Question about rpm issue

So my 330i while driving without cruise control on, the RPMs go down and then up at about 400rpm intervals, it get really annoying.
I was beginning to believe it was torque converter, but I noticed that whenever I use the cruise control the car drives perfect without any RPM fluctuation whatsoever.

Any idea what can be causing this? I have no codes, I already scan the whole thing with PA soft, INPA and nothing there either.

-Can this be caused by:
A-Throttle Body?
B-Transmission Input and Output speed sensors?
C-Anythibg else you may believe to cause this issue?

Thanks in advance.

Question about rpm issue