Your center plate is worn out and also a couple of your bearings are worn out. These two combine to allow the mainshaft to "walk" or move forward or rearward depending on the direction of the load, and since the mainshaft is moving but the shift fork is not, the movement drags the mainshaft under the shift collar/hub thus disengaging the gear.
After you notice this in 1st, as it gets worse, I believe it will also begin to pop out of 3rd gear also.
The only way to fix this is the replace the worn out parts.
BigMike
