Baumatic B12 hob : intermittant problem…

Is there an inherent fault with the Baumatic’s B12 ceramic hob?

For some time now, our hob continues to throw a strange wobbler : to start with, on odd occasions the smaller diameter ‘cooking zones’ failed to work…but the next time we tried it, they worked just as they should.
After a few weeks, they just stopped working completely.
It’s always the same pair of ‘cooking zones’ that cause the trouble – the 1.20kW smaller rings, top left and bottom right as you look at it.

We had an electrician friend have a quick look at it last week : he couldn’t find anything wrong at all. The wiring is “logical” according to him, with no burnt wires, loose connections or dry joints in evidence.
In effect, everything is as it should be.
As a matter of course, he tightened the connections, before he put it back together, positioned it in the worktop cutout, et voila – everything worked!
All four ‘rings’, absolutely fine!

24 hours later, the hob was used again, and all rings still worked!
Success! ….except that only 5 minutes later, the two culprits packed in again!
Is there some kind of ‘safety cut-off’ that’s mistakenly cutting in, or……?

All suggestions welcome, and my thanks to you in advance….