Zanussi hob, sometimes working sometimes not. It needs a small hit?

Hi,

I have a Zanussi ZKT 621 LX/LN ceramic glass hob.
I bought it second hand and the owner said that left 2 rings are sometimes not working. I thought that I could live with that as long as right 2 rings are working very well. However, right 2 rings started not working properly recently. For example, if I set it 9, it starts getting red and after a while it stops. Then never starts again. I gently hit the glass surface of the ring and it started again. Somthing like knocking the surface. It is not a coincidence because I did it after couple of minutes after it stopped. Then I decided to open it and see if there is a micsonnection or broken cable and stuff. I didnt see anything wrong. I tried it and I saw that left 2 is also working (they were not working before opening the case). Right 2 is working as well and I said myself that I fixed it. Till this morning. I set it 9 and put some water on top of if. I let it boil some time and come back to check it after a while. I saw that the hob is not heating. I hit the surface again 2 or 3 times gently and it started working again.

After this long story, could you please tell me what the problem is? I can do some soldering if it is needed. Just tell me what should I check.

Thank you.