The plastic shifter seat and bushing are most likely shot. I see this all the time at work, usually AFTER a transmission shop has told the customer that they need a new synchro or somethign stupid. Basically, if these parts are worn, they won't cause the slider to engage fully on the hub, and it pops out of gear. Usually first gear, but sometimes 3rd and reverse too... They're cheap and easy to replace.
Check this out: http://www.marlincrawler.com/htm/transmission/trans_parts.htm
Ryan