Tubby
You specifically refer to non FADEC engines.
Such hydromechanical fuel control systems are very complicated with multiple half ball valves. Each of these components can fail - often due to fuel contamination with dirt, swarf etc - and the result in each case will of course be totally type specific.
In my experience such problems can certainly lead to a rundown to a sub idle condition. Opening the throttle will have no effect unless the reason for the rundown coincidentally goes away. Equally a shutdown and relight will not help if the fault causing the rundown still exists.
JF