Baxi Combi Instant 105e - Pressure Gauge?

My Baxi Combi instant 105e Boiler Gauge keeps dropping to zero pressure, after repressurising and during operation. It still provides hot water, but the temperature drops in & out.
It has been recently serviced and the pump was changed. The Engineer was also called back to look at the issue with the pressure gauge.

Any advice out there please.