How to power a well water pump on the local electric grid
A dramatic sag like that, when you add a big load, is due to one of two things. We'll look at them individually.  

Voltage drop

Voltage drop is caused by resistance in your wires and connections.  Resistance is normal for wires, and is a function of their size and their length.  Bigger wires have lower resistance, and longer wires have (proportionately) more resistance.  Long runs like 30m or longer have significantly more resistance, and can necessitate using a larger wire.  Since you know amps, voltage drop can be calculated based on wire size and length; there are calculators all over the web.  

It may seem economical to use thinner wire, but over a long distance it can stack to voltage drop of concern.  

Connections Are not supposed to add resistance, but a bad connection will. You can spot one because it will be warmer than the wires around it.  That is BAD and not normal, and should be fixed at once.  Though if that connection is causing a 90 volt drop at 13A, that's 1170 watts which would burn up in a hurry! 

Lost neutral

This voltage drop is so extreme (and labile) that it might not actually be voltage drop.  That has me think it could be a phase balance problem from your supply. 

In 5-continent power, which follows Europe's design, power is distributed to the neighborhood in high voltage, and then the neighborhood is supplied 400V 3-phase "wye".  Neutral is in the middle, and is tied to earth for the single goal of keeping the three phases only 230V from earth, which is safer.  Each house is provided one phase and neutral. 

The various houses are randomly given one phase or another, so on average, the three phases are fairly well balanced.  Your power use returns via neutral, but it's quickly distributed to other houses if loads are balanced.  That's never perfect, and that's why neutral is attached to the supply transformer.  This transformer tie forces your voltage to "stay in the middle of the wye" - at 230V regardless of the other loads at your neighbors'.  

But what if neutral went away? Then neutral would not stay in the middle.  It would drift/float all over the triangle defined by the phases.  

Then the three groups of houses on the 3 phases would be like an electrical, Y-shaped "see saw".  As one group of houses loaded the circuit more, it would make its own voltage sag, but the other houses' voltage go up.  This see-saw action would happen instantly as loads changed.   We just had this happen at my complex, and making toast would sag our voltage 20 volts (out of 120). 

This, most likely, would be a problem at your utility, probably right at the supply transformer.  You can detect it two ways, first by measuring power at your service entrance (thr demarcation point between utility power and your wiring), and second by asking your neighbors if they've seen too-high voltage or had appliances fry on them.   

You might be able to follow the wires on the electric poles, and see if you can visually observe which phase your wires are connected to.  

Remedying it

If the voltage at the service entrance sags about the same as the numbers reported on that transformer, that is a lost neutral (or other supply problem, such as too-small wires on the poles).   If the numbers at the service entrance remain good while pump voltag sags, that is your wiring.   

The lost neutral problem could only occur at a point common to you and your neighbors, so it is up on the pole and in the responsibility of your utility.  

If the problem is voltage drop, then crunch the numbers and see if the voltage drop from your wires will explain it. (If so, what to do about it will be obvious.) If wiring drop does not explain it, then examine each connection - be careful not to burn yourself on a hot connection!