[Mk2] [Turbo] water gauge goes down when accelerating ??

[Kubus]

hello - i took my car for a ride ( first of all it didnt boost as should be once went to 11.5 psi and after that to 8.1 psi.... should be 15 at the moment )
when accelerating i looked on the gauges and the water temp gauge went down to zero ..... when took my foot of the pedal it went back where it should be.... what can that be ??
i am confused at the moment....

[raptor95GTS]

thermostat is likely broken. Are the front rad pipes getting warm, do you get hot then cold air from the heater when this happens?

[Kubus]

will check it tomorow ......thanks mate.....