Beta 87 fault quick question

Hi I know there lots of wise heads here wondered if I could have your thoughts....

My beta 87 works fine for 30 secs of a minute or two then audio fades to nothing..... if I un plug plug back in it works again full level but again for 30 secs to two mins again....

I have cleaned the board re soldered connections.....if I take basket off I hear the usual hum and the hum fades away with the audio....so I think capsule its self is ok....

any thoughts would be very welcome thanks

Photos of the circuitry?

Hi Khron thanks for response sorry I been out singing for my money... got no pics at moment poor mic not in great shape it the FET died years ago was replaced with a 2N3819 am I right in saying tho wrong FET should work & wouldnt be the cause of the audio fading..... or could it be.....