I do also suffer from bad reception from beginning with my FP1 as you can read here.
Mostly it happens in areas with bad reception at all but then either the call is interrupted or at least the other don’t understand a word during a call, or without a call the bars are jumping between 0/1 and 3-4 bars.
I think it is strongly dependent of the three-dimensional orientation of the phone (and its antennas) in regard to the sending masts…a 180° turning movement can provoke it in some of my cases!
But I do not have a solution, only in my flat I know how I should hold the FP to have acceptable reception quality.
Maybe you can play around with the type of network: try only 2G or only 3G and compare to the automatic mode of 2G/3G-mix. You can achieve it easily with XPosed+Gravity-Box.
I am using a two year old Fonic(O2) SIM.
Cheers, Robert