Bumping out of 2nd

I would suspect the keys (or struts) that are worn; they and their associated springs have the job of preventing this. They set in the slider and are part of the 1-2 synchro assembly.

Looks at the parts numbered 13 in this diagram (they keys), and the PN 11 and 14 springs.

http://www.tbtrans.com/a833_4_speed_transmission_pa.htm

I have replaced these in Opel and Muncie 4 speed trannies to fix this exact problem. And yes, the synchro ring itself is not this issue's cause, though I suppose one could say it's the 'synchro assembly'....; I can't say yes or not on the worn parts mentioned.