Really challenging problem with daikin outdoor unit.

[NoGrip]

Hello everybody, first of all, sorry for my English.
i'm a daikin user experiencing a weird problem with a 4MXS68f2V1B outdoor 4-splits unit.
Here it is: after a completely random time of operation (from 2 to 8 hours) the outdoor units stop, the indoor unit goes to ventilation mode as nothing happened, no led blinking, no errors neither on the internal unit neither on external.
Then after some minutes the external unit restarts, but just to stop after less than a minute. Then it goes on with this cycle indefinitely (sometimes it stops, and for some hours it operate correctly).
Obviously the performance is terrible: the temperature goes down and up weirdly.
I called the local official technical support, and a year ago they suggested PCB board replacement (very expensive repair), the problem seemed to disappear, but, now, after a year it emerged again.
I called the technical support, they checked all the unit and just said: "no error = no problem" because obviously they can't stay here 8 hours to wait that to appear.
I think that it is a problem probably related to Under-Over voltage protection, but i'm not a technician.
What do you think?
Thanks in advance for you help.

[frank]

A mains supply over or under voltage will stop the unit on a U2 error
Have you had the gas charge checked?
Is the total length of pipe to all the indoor units below 75m?
Does it run through the Test operation OK?
When the green light is flashing do you check for fault codes with the remote controller?

[install monkey]

your english is better than mine and ive been here 40yr!:p
when the when your unit is operating normally check the voltage on 2 and 3 on the outdoor unit to see the comms voltage pulsing- then when it plays up check the comms voltage is the same,
4mx units have a tiny pcb next to the terminals on the outdoor with lil led's- ensure 1 dipswitch is set for dedicating 1 indoor unit as a "master"
http://www.mediafire.com/view/nj11wv8d8awpxty/4mxs80_mulitsplit_service.pdf

[NoGrip]

A mains supply over or under voltage will stop the unit on a U2 error
Have you had the gas charge checked?
Is the total length of pipe to all the indoor units below 75m?
Does it run through the Test operation OK?
When the green light is flashing do you check for fault codes with the remote controller?
Gas has been checked, other parameters too, by the techincians.
They done the test operation too.
The green light just flashed a couple of times before PCB replacement and i got U2.
Since replacement no more errors (neither external or internal unit) but still i got the same problem.
From the daikinf troubleshooting:
Operation stops unexpectedly. (Operation lamp is on.)
This is not a malfunction of the air conditioner.
When the voltage fluctuates largely, operation can stop to protect the air conditioner. Operation will be restarted automatically in about 3 minutes.
I think that's what happening, problem is that happens so frequently that it can't manage to cool the temperature.

[frank]

Then I would think you would benefit from a voltage conditioner from APC
http://www.apc.com/tools/surge_selector/index.cfm