There are three (generally speaking) things that can be causing this problem, Mark. You could have a problem with the clutch, the gear selectors or something internal in the gearbox itself. A worn or collapsed bearing or mangled gear cluster could cause this, but I’d expect other symptoms like horrendous noises and the refusal to select some gears at all, not just randomly. If you’re lucky, the cause could a simple mal-adjustment of the selector mechanism.

Meanwhile, you’ve told me you don’t think there’s anything wrong with the clutch but, to be honest, I’ve seen these same symptoms before on cars with worn out clutches. Sometimes the actuating fingers of the clutch break, other times there’s just too much wear for the clutch to function properly. Sometimes it’s as simple as a leak from the clutch’s hydraulic system or a stretched clutch cable. But either way, clutch problems can certainly cause this sort of grief.

Reverse can be hard to select because the clutch is not disengaging fully, so try this experiment: Turn the engine off and try to engage reverse. If it goes in easily every time and only baulks when the engine is running, that’s a classic case of a dying clutch.