Bosch Maxx WFL 2000 only drains intermittently – pls help

Hi folks, i’m after some sage advice – here’s the situation:

-sometimes a cycle fails with the end light flashing, and water left in the tub. Sometimes it’s OK though and completes
-the pump makes a strange humming noise
-the impeller in the bottom right service flap is clear and spins fine
-the exit hose and drain pipe it feeds into is clear
-when it fails, it empties most of the water, but not the last gallon or two
-if i blow into the exit pipe when the pump is humming and it’s trying, but failing to pump out, it suddenly fixes it and the water is pumped out without problem
– yesterday i did 2 90c empty washes – one with vinegar, and one with some dishwasher tabs. a fair bit of goop is still coming out when i run empty rinses

So
– do i need to get the pump out?
– the manual suggests a blockage after the pump – is this likely?
– is the service flap impeller part of the pump, or is there another impeller?

Many thanks