Pump pulsing or running for a long time

Indesit EDIFP 28T9 dishwasher

The cycles run OK. But whenever the drain pump goes it runs constant then starts pulsing - every second it gives a push, then eases off, then pushes, then eases off, etc. Sometimes briefly, sometimes for a long time. Sometimes at the end of a cycle I have to switch off the machine to stop it pulsing !

When I turn off, I hear a gurgling sound from the sink drain and notice the sump is filling back up with water.