The usual problem is worn shifter forks. When the shifter fork gets worn, it doesn't fully engage the gear, and that causes it to pop out. The tranny has to be repaired to fix it. At that point you usually replace the gears also because, most likely the popping out has caused some wear on the outer edges. While it's open, a full inspection may disclose other parts that are worn and should be replaced while it's on the bench. You could get by for a couple hundred dollars if you just replace the fork and 1st gear (maybe the synchro too) yourself, or $1200-$1500 to have a full rebuild done by a pro.
Jon Logan (
http://www.midwest124.com/) recdently did an excellent full rebuild for me for $1200 plus core charge if you need a recommendation.
Ron