The water should constantly run to the expansion tank, even on idle revs. There might be some obstruction in the lines but I would reccomend to turn to the coolant pump first. Is it leaking? Is the coolant fluid dissapearing in small amounts sometimes? If yes then the coolant pump needs to be changed, which can cure your problems.
When the enigne overheated and the 3rd cylinder is only on 90 psi (6 bar) and others barely on 120 psi (8 bar) it means the head gasket is gone or the bore area is very worn. Having used the car under heavy conditions I would say the bore are is hard to ruin, so turn to the head gasket.
The cylinder pressures should be 160 psi (almost 11 bar) with gas pedal to the floor and should be even only 1-2 psi difference.