Transmission shifting into low

What is your question?

1998 chrysler sebring Conv. v-6, when I am traveling up a mountain road, about 15-20 minutes into the 1 hour drive, The transmission will shift in low gear and will not come back out of it, til I pull over, put it on park, shut it off for about 10-30 seconds, restart it and drive off. I works find til it does it again sometimes 3-10 minutes of driving, Does not have this problem while on streets/freeways. What's going on here???????????????