Ferroli Modena 80E boiler is heating water outwith timer periods?

Ferroli Modena 80E boiler, about 7 years old. Various problems being encountered over the last 6 months:

No DHW indicator on when running a tap.

Initial burner active after using tap but turning off after short period (less than 30 seconds). Eventually after stopping and starting the tap running there is a good chance it will remain with the burner on.

Boiler fully active for short periods (less than 5 minutes) outwith the times specified for the timer circuit.

I have checking the external timer circuit and room thermostat and everything is correct. (open circuit when timer or thermostat inactive and vice versa)

The last time an engineer was out the system was behaving itself! On that occasion it was serviced and stable operation was achieved for a couple of weeks however the issues re-occured.

Given the range of problems being experienced I am inclined to suspect the main pcb is faulty.

Any advice you can provide will be appreciated.