Altitude wildly erratic but baro pressure ok

I have three pixhawks and they all sometimes display very erratic altitude estimations, although absolute pressure is fairly stable, I have one recent tlog covering 40 minutes sitting on the bench, and I rebooted the pixhawk twice. In the TLOG, the alt_vfr_hud_t varies between -79 and 117 metres,

Sometimes on of the planes boots up and has the correct altitude, remaining about 0, but occasionally it goes berko. The other two are wrong more often than not.

Any help welcome, this is urgent