Espresso machine has to warm up halfway through pulling a shot
Why do you stop in the middle ? Is there a drop in pressure ? does the coffee continue dripping down in the cup?

Let it finish.

I assume, if it is working like my machine, that the water is being re-heated for a next shot.

1st thing I would do is to clean the machine with a "descaler"; it should help with the water heating system and the pressure.

If that does not help, have the machine looked at.