BEKO Dishwasher problem

I have a BEKO DWD-4310 diswasher with the following intermittent problem. On a successful wash cycle you can hear vigorous washing action for quite a while, before it goes into a secondary cycle with the rotor arms turning slowly and it starts heating. The fault is that it seems sometimes to skip this initial wash and go straight into the secondary cycle with slowly turning arms. The result is dirty dishes and often the remains of a dishwasher tablet in the bottom of the machine. If I indentify that it’s gone into the faulty mode I can usually get it back by forcing a pump-out (holding the button in), then doing a rinse cycle, then, when that has started successfuly (vigorous action), pumping out again, then re-setting the wash cycle and running it again. This time it usually works. This sounds like controller problems, but could it be sensors? Does anyone have any ideas? Thanks!