Welcome to the MMO.
Under what conditions does it pop out of 1st gear - under acceleration, while coasting along in traffic, or when decelerating? And what happens if you try holding it in gear - a regular clonking noise probably means a tooth missing off the 1st gear cog.
It may just be a simple case of a worn remote gearchange linkage - does the gear lever feel very loose and floppy?
Alternatively, it may be a loose bolt securing the 1st/2nd gear selector fork onto the sliding selector rod - remove the side inspection hatch to check the tightness of the 7/16" bolts that are used to secure the forks to the rods; they do sometimes work loose, resulting in the gear not being quite selected fully.