Good afternoon, my name is ***** ***** I will assist with your query today.
This sounds like a fault with the power regulator, which is responsible for taking the mains power, passing it through a circuit and then feeding the relevant circuits to power the microwave.
I have seen this alot, and is a reasonably common fault, due to the high current required for a microwave to function.
Before suggesting a replacement power board and looking up availability of parts, can you confirm that as part of checking the power supply you have inspected the microwave cable and there isnt any obvious damage to it and that when its working if you tap the plug it doesnt go off (just to rule out a cable or plug fault).
The parts do not appear to be readily available, for example the online store here only appears to have the high voltage side of the circuit.
Realistically I think it will probably be the capacitor or diode at fault, but with it being intermittent it can be a challenge to fault find. However as this is a high energy appliance I would certainly recommend using a service centre rather than attempt the repair yourself as your dealing with lethal voltage levels
The parts are likely to be around £30-60, depending on which part of the power circuit has failed, if you can provide your postcode I can give you a rough idea on labour charges
A mobile engineer could repair the the microwave however this normally attracts a significant call out fee.
I would recommend using D.A.S they can come to site or you can drop it off to them, their number is ***** 301543, alternatively if your happy to drop it off you could use Stuart Wallace (01522 791496), however I know they have a a week backlog currently.