I have intermittent problem with two of the four rings - sometimes they work sometimes they don't

See this causes and proceed for fix it. God bless you
Surface Element Switch If the surface element won't work, and there is another element of the same size on the stove, try switching the elements. If it still won't work, the surface element switch is probably defective. The surface element switch can't be tested or repaired, it will have to be replaced.

Surface Element Board If a surface element won't work the surface element board might be defective. Each burner is switched on and off with the help of a relay on the surface element board. If one or more relays has failed the surface element won't work. If two or more burners fail at the same time, this is likely the problem. If only one surface element won't work and the element itself checks out to be good, the surface element board might be defective.

Coil Surface Element If the surface element won't work, the coil surface element might be burned out. Most coil surface elements can be pulled out of their socket and checked with an Ohm meter for continuity. If the surface element won't work, check to see if there is any visible damage to the coils. Also, check to be sure the element is pushed all the way into the socket.

Radiant Surface Element The most common cause when the surface element won't work is the radiant surface element itself. The radiant surface element - or heating element - can burn out similar to a light bulb. If the surface element won't work and it doesn't have continuity, it will need to be replaced. The radiant surface element is not repairable.

Solid Surface Element If the surface element won't work, and the stove has the solid surface element type of heat, it's likely that the solid surface element itself has burned out. The solid surface element works very similarly to other surface elements - inside there is a heating element that glows when electricity is applied to it. As it gets hotter, the heat is transferred to the solid surface element

Terminal Block If a surface element won't work, there may be a problem at the terminal block. The appliance has a power cord which is attached to a terminal block. Over time, the connections at the terminal block can fail and burn open. When this happens, normally all of the surface elements won't work. The solution is to replace any burned wires and replace the terminal block.

Loose or Burnt Wire Connection If surface element won't work, there might be a burnt wire supplying power to the surface element. Sometimes the wire supplying power to the element burns out right near the element itself. Check for this by just looking at the wires leading to the element. If they're burned out it is usually easy to spot.
Incoming Power Problem Although not as common, if the surface element won't work it is possible that the appliance is not receiving proper voltage. Electric ranges require 220 volts of alternating current. If for some reason the appliance is receiving significantly less, the surface element won't work. Check for proper voltage using a volt meter at the socket where the appliance plugs in.