Modern Vespa

Not long rebuild a Vespa p200e engine and I'm having problems with 3rd gear slipping out and back in. Happened 3 times in 45 minutes of riding. I'm thinking it's the cables too tight or too loose hopefully. How tight or loose should the cables be.

Selector setup/cables- this may help: http://vespamaintenance.com/engine/select/index.html

If adjusting the cables does not resolve your problem, here are a few things that may be the issue:

Worn out shifter box

Worn cruciform

Run out too large-increase size of shoulder washer within spec.

I did the cables but it's still the same jumping from 3rd to 4th and back to 3rd. I broke down the clutch didn't work so I had a look when I got home and found a part in the clutch cover was broken as you can see in the photo. Could it be that part doing it. Or maybe it's a shim problem I'm thinking now because I use a drive shaft from a p200e I had years ago so it could be the shim. The gears are new.

Great catch, but that will have no impact on your gear issue.

The P200E and PX200E don't use compatible components. If it has a flat cruciform, it needs the one piece rod and EFL selector box. If any of these are mixed, it will jump gear.
Jumping gear can crack the engine cases.

didn't you also just change your cross without splitting the cases?

was it jumping and you decided to do the cross and now it's still jumping?

as to the driveshaft, like jack says there's not to be any cross pollination on the components. while shimming certainly can be part of the cause, it sounds to me like you've got deeper issues.

I'm guessing you're gonna need a 3rd gear and a new cross at minimum.

Also, the broken part in your photo is the little part that sits underneath the brass plunger in the cover that pushes on the pressure plate. You'll have to take the spring and the arm off the cover to replace it.