1989 Toyota Corolla 5th gear slipping into neutral increasi

My 1989 Corolla started occasionally slipping from 5th gear into neutral this July, and it has started doing this with increasing frequency in the last few weeks. It occurs generally when I'm forced to slow, sometimes just a very few mph, and yesterday it did it three times back to back as I climbed a nearby ridge. The auto parts guy said he'd had a similar problem, and it was unfixable, or perhaps just too expensive for a car I paid $1800 for 7-8 years ago! Am I stuck without a car or just paying 4 times its Bluebook value to get it repaired?