Intermittent Gear Selection Issue.....

[Oldman]

Hi all, hoping the brains trust can help me diagnose this issue......
Every now and then, when I try and select reverse, I get a clash of gears.
There is also an intermittent "locking" in gear in random gears......ie it can happen when I try to shift up or down...
This happens infrequently and not all the time......and it's bloody annoying when it does. 
If I turn the engine off I can select, shift up & down gears without issue .

Initially I thought this could be simple clutch cable adjustment so I dealt with that....
No joy.
Then I thought front shift bush.....swapped it out for a new one....old one was fine.
No joy.
Also checked the rear shift coupling…..it’s fine.
The gearstick mechanism (Vintagespeed) is in as new condition and without distortion.
Shift plate has not moved since I installed the VS shifter, so I've discounted that.

I am starting to think that there may be a bent guide fork in the front of the gearbox.....though I’m not sure how that would happen…?

Any ideas would be greatly appreciated, thanks in advance.