Flight director pitch up

I have a C172 with 2 G5’s and a GFC 500. On three separate occasions while flying a coupled approach the FD indicated a 2-3 degree pitch up while on the glide slope and the AP followed.  We were dead on the GS. I disconnected the AP and hand flew the approach. This also happened in straight and level conditions with AP engaged. Anyone else experiencing this issue?

Thanks in advance