Not so sure anymore...

• those glitches do not happen at regular times
• I tried to send a sinusoid into another ADC channel, and I couldn't see any jump
• using DTT and zooming into the jumps, they don't seem a clean one-sample jump anymore... There might be a bit of ringing before and/or after the jump
• The jumps have different sizes and directions, and they seems to happen at the same time in all four quadrants, but not in the same direction or with the same amplitude

I was suspecting dust crossing the beam, so I build a very rough enclosure, that should help with dust. I don't think I saw any change in the glitches. 

Also:

• glitches are there even with the HeNe off, with ambient light only. So it's not the laser.
• I tried sending a sinusoid into one of the same ADC channels used to acquire the QPD signals. I couldn't see any glitch

So one might conclude that the glictches are produced by the analog QPD electronics. However, I plugged in a scope and I couldn't see any in the analog signal. But I checked only before the DRV135 stages. I'll need some sort of breakout board to test the output of the DRV135.