3 rd gear change not ok

Hi on my t5 some times when you change from 2nd gear to 3rd it does not go straight in or some times it will go in then as you wind the throttle open it slips out all ways happens when there are people watching the cables seem ok it never happens when changeing from 4th to 3rd had new cruxiform bout 18 mounths to 2 years ago.It goes in to the other gears ok could it be time for a new gear selector box Thanks Gary

Sounds like the cruciform again, as it’s almost always the 2nd to 3rd gear that jumps, just when your putting on the power!
The t5 and later 125/150/200 has a better layshaft(grooves cut into the layshaft which hold the cruciform in place) the trouble I have found is that a lot of people get given the wrong parts off of earlier gearboxes which don’t help!
To start make sure the cables are not to tight that they are pulling it out of gear.
Then try looking at the selector box, at the arm that holds the cut plate(has a roller that sits in the cutaways) and see if it moves,(moving the gear change on the headset slightly backwards and forwards) as that wears, stopping it holding it in gear,the arm wears and puts grooves in the holding screw(you can get a repair kit ,the arm/and small screw, but some slector boxes you can’t!!) also make sure if you get a selector box ask for T5 as earlier px/125/150/200 selector boxes are diffrent,(makes life easier, as the T5 has always had this!) as the arm that moves the cruciform has a 2mm diffrence.
And also check that the arm that the selector box arm fits too hasn’t unscrewed it’s self(reverse thread, 17mm spanner tight but not too tight as the threads stripp)
but if its jumped before might just be worn gears[:(]
plus! was the cruciform flat? and no washer?

Thanks for reply will take look today and see if i can find problem