Thank you Sylvain for your prompt response.
Yes, I applied the source orientation after averaging time series across the atlas parcels.
Although, for the sake of clarity, the plots attached above refer to the PTE in the frequency range 1-4 Hz and already at low frequencies one can notice a strange response.
I will try the two options you suggested. The 2. proposal is computationally too expensive even on a cluster. For this case I have to reduce the size of the matrices.
Thank you!
Best regards
Max