Modern Vespa

The shifter on my '64 Allstate takes a good bit of torque to change gears. Not excessive but not smooth at all either. I have rebuilt the selector box, replaced all of the cables, adjusted the cables so that there is about 3-5 mm of play in between gears at the shifter and replaced the gearbox oil with full synthetic GL-4 75W-90 oil. Before I pull apart the handlebars for a clean and lube, is there anything else I should be investigating? Thanks.

Sticky shifting can be caused by badly routed/poorly adjusted cables, worn or dirty shifter parts, a worn/rusty handlebar tube, or possibly a worn or bent output shaft ...

You can isolate the issue by disconnecting the shifter from the engine with the cables attached and see if you can shift it smoothly through all the notches. If not, disconnect the cables, and try clicking through the notches using your thumbs and moderate pressure.

If the shifter works smoothly when it's disconnected from the engine, check that the outer cables have their top hats and aren't moving in the headset.

^^^
Good info from SoCal as usual.

I had a PX200 at one point with similar issues and it was infact bad cable routing.

If it's notchy when riding it, and nothing else is apparent, then most likely the clutch is badly worn.

In addition to what Socalguy said I must add frayed cables internally. I recently replaced 5 or 6 cables within a span of a few weeks and I found everything, from frayed cables to even detached/stuck teflon liner inside the outer.