On one of our old 320s (One that has been around since early 90s) Sometimes I get them together with Both FACs failing due to the power interruption if I don't switch them off prior to transferring the power from the external power to the APU.
And the fault never clears even if I depower then repower the aircraft, So what I do is I pull out B11 A01 A02 R20 wait for 5 minutes and then rerack it then reset them and the fault clears, its quite irritating especially when the crew start calling us for it right before departure.
I'm guessing It's due to the computer ageing and hence the quality of it's power supply circuit plumets down but we have not tried changing the computers because it's an intermittent fault.
I believe you're dealing with an old aircraft and this happens due to the
engine generators to APU power transfer.
Otherwise If resetting the hydraulics only fixes the problem then I'm assuming that there is something wrong with the high pressure manifold pressure switches (switches are sticky or something) such that when the hydraulic power is transfered from the EDPs to the PTU and yellow electric pump then that results in the problem.
Thats off the top of my head.
Have you tried replacing both ELACS and the pressure switches?