Hot Water trips Boiler Worcester 28i junior?

My Worcester 28i Boiler works fine most of the time, I have had the PCB changed recently as advised when my boiler failed, it has now started to occasionally trip out and the red light flashes about 4 times a second. I had a plumber out who changed the PPS it worked ok for a few days but it still trips out, predominantly when we run our Aqualisa shower, but it can be tripped when just turning on the hot tap. When we press the reset it always comes back on the heating works fine.