Slipping in 2nd and 3rd

[chevrons1]

Morning, as the title suggests I am slipping in these gears. Feels like it is going to jump out of gear but doesn't. When I get to 4th it is all fine.

Any clues?

help me obi wan........

[henri]

1st check ya cable tensions ,1 cable too tight other loose, can be trying to pull it out of gear ,as its ok in 4th i'd suspect that cable ,the outside 1 on selector box .after that it gets more complex ,weak spring on selector pawl on top of selecter box or worn pawl/quadrant so its not being held in the indent for the gear you select . or damage/wear on the cruciform inside gearbox, thats for jumping gears .
but slippings usually more a clutch issue , overtight cable adjustment =not enough free-play at lever can mean when powers on in lower gears clutch lets go n slips ,but cruising in 4th it hangs on. or worn out clutch plates an weak springs . H

[pxguru]

If by slipping you mean quickly jumping out and back in again on it's own, then this is a worn gearbox.
You might be able to save it for a while. By the time the "slipping" starts, as H says everything is worn. It's not normally one thing but everything!
To save taking out the gearbox for a while, fit a whole new selector box or if you're tight just refurbish kit with new pawl and spring. You are going to have to do this anyway so by doing it now it might temporarily stop the jumping, just for the Summer riding season at least.

[chevrons1]

Oh lord! Trading in for a new PX150 gets ever more tempting!

[henri]

its always tempting , but you'll regret it an feel forever guilty . this is how they make us poor suckers love there fickle little asses . scoots , ya got to love em , only other way is straight thru the madhouse door . H

[pxguru]

Its not going to be that expensive! Just a whole days work. If it doesn't need gear cogs, then less that 100 quid for sure.