Mystery Engine / Gearbox problem

[Quintessa41]

I have a Perkins M50 with 2 yr old ZF12M trans . The injection pump & injectors were reconditioned 6 months ago. Engine & trans working perfectly
“However “ occasionally after many hours of motoring at 1800rpm , I idle down and select neutral for a while and then move the lever to forward or reverse I cannot get any more revs than idle speed . If I shift to neutral I can rev the engine to any rev speed I like then put it into gear again and it won’t rev above idle ?
But !! When I do this in gear the throttle lever on the pump moves and should rev up but it doesn’t. Select neutral again and it revs perfectly. After idling for say 30 mins everything’s back to normal again?
The Morse cable is not slipping I can see it working perfectly in gear and out of gear. I can see the the throttle lever on the pump working perfectly in gear or out of gear . I think it’s heat related to the trans ? The problem disappears as soon as the trans cools a little. Has anyone had any similar problems I’ve had this problem looked at by mechanics but no one can figure it out .
Any cluey mechanics out there ?

[skipperpete]

A sticky feathering prop can cause this. Do you have a feathering prop?
Also a chance that a hydraulic shaft brake that drags due to low oil pressure at idle could do it but that would involve smoke and probably noise.
The gearbox being faulty is a possibility but I imagine the oil would look black and smell burnt if the reversing gear had enough internal friction to hold an M50 at idle regardless of throttle position. [emoji848]