Help on fault finding on a C3-31 Hammer combination machine

[OMLCNC]

However, I still suspect the real problem is the supply voltage sagging with the starting load.
What happens to the main phases powering the control circuit (L2 & 3 on the diagram) when everything is starting?

Disconected brake by pulling off C1 and SP terminals and insulating

So I took multimeter set on Hv put one end in earth and other in L1, L2, L3 in turn. (at the overload 4)
L1 Black wire 330v no load no dip (because the contactors stopped chattering while testing?)
L2 Brown wire 120v no load dips to 75v on start
L3 Grey wire 361v no load dips to 330v on start

While testing on and off and rotating switch for different motors, the chattering contactor problem stoped and I canot get it to do it any more.
There are small sparks from the contactors 5 and 6 on switching on and off.
I dare not touch anything as the problem may return.
What should I do next?
Go back and see if It is still ok, then re-connect the brake?