Baxi 100/2 HE Plus burner keeps switching off?

my Baxi started to go into lock out, plumber found condensate tank full, drained and cleaned all ok, had it serviced following week, 2 days later boiler started to go to power on, burner on after about two seconds the burner would switch off the start again, over the next two weeks the pump and 3 way valve changed and when the fault came back a couple of days later the flow valve was changed.It now works but takes 2 or 3 attempts to light and still sometimes goes to lock out.....help!