Hello

When it jumps out gear its likely to be a transmission selector hub with a damaged detent spring, or the detent ball bearings are sticking in the hub and not locking the selector ring in place or the other less gloomy possibility is the linkage is poorly adjusted

there is some adjustment on the base of the gear lever so try putting it into 2nd and then loosen the bolts off at the lever base and see if you can move it any further into gear

if you go too far you then have issues with it jumping out of 1st so you need a sweet spot between the 2 positions

also lift up the back seat cushion and remove the little access plate on the tunnel this should reveal a two part linkage for the gear lever rod there rubber bushes on these can wear badly and make the change very sloppy so check they're all in in good condition

Hello

yes the base is at the bottom of the shifter

see here for a good how to

http://www.huelsmann.us/bugman/ShifterTech.html