Baxi Genesis 80 - What's the fault?

Sometimes it works properly but most of the time on DHW it ignites, burns for a very short time (1 or 2 seconds), goes out, re-lights, goes out, and keeps repeating this. On CH mode it ignites, burns for a bit longer (a few minutes) and then cuts out with no more than 2 or 3 LEDS glowing and rads only warm, it then takes a long time before it re-ignites and goes throught the same process.

The 3 way valve, flame sensor, spark ignitor, pressure switch, prv, safety thermostat and pcb have all been replaced. The system is holding pressure. Repairs are costing a fortune!

Any ideas?

Thanks.