Manual trnas won'tstay in 1st gear, unless?
 

2008 Saturn astra xr manual transmission: Problem is that after vehicle is running try going into first gear and as soon as let clutch out it pops out as if it isn't in gear all the way, if shut engine off can fell it shift and can get first gear and works ok, sometimes if you hunt for it you might get it to go into first while running. But not too often, usually shut engine off and find first, yes feels like different when you find it, looked at shift tube, the linkage the engages the shifter rods and that funky setup has a plastic connecting rod which appears to be worn, thinking it could be an issue, anybody have apart number or where can i even find these parts

Also sounds like it could be the slave cylinder.
good luck finding parts for an astra in the states. May have to go the Europe route for an opel astra. Could also try junk yards.
I made bank off a 5 speed astra I found at the junk yard a few years back picking that thing for people all over the internet.