One FSU repair person note about getting FED-3 re-hauled was ... "you'll need nerves made from steel, but if you will make it, camera will serve as reliable".
Have you lens off and looked inside? It might have explanation for the problem visible. And it might sounds like re-haul.
Oh, FED-3 isn't as bad as Zorki. You could change shutter speed at any time.
Link to the manual to prove it.
First sentence on page 8. But it is in Russian