Ferroli Modena 102 luke warm hot water?

Any help have Modena 102 sometimes when 1st time calling CH the gas flame is very low if flame doesn't spread to flame sensor, boiler shuts down. If flame does spread across burners and sensor is ok flame stays low for 40 sec then modulate all functions ok & for the rest of the time the boiler is in use for CH & DHW, until shut down over night. Next day same scenario. Its same with DHW 1st call after boiler being off for a long time overnight or more once I’ve managed to get it fired up all function are ok, Normal procedure is to select CH if flame does not spread across burner switch dial off before shut down mode try 3,4,5 time till flame spread now all ok. Boiler has 2 thiristor sensors fitted 9 months ago, checked reading on both in situ when boiler off overnight 1800kohms & 1736kohms. Have had fan & connections cleaned & checked, pressure switch and connectors cleaned & checked, no blockages in venturi & pipes ok, over heat sensors & connectors checked and new heat paste, direct cold water in flow meter filter cleaned & impeller spinning ok have removed plastic flow restrictor due to low water flow and pressure, heat exchanger removed and flushed out rad flushed out, boiler pressure constant 1.5 bar, direct water in 2 bar dropping to 1.1 bar when hot taps on. Any ideas

Bill
November 2012