1994 Saturn Station Wagon

When I start my car and it is cool every thing is ok but once the engine starts to heats up it wont shift into reverse. And when it gets to the normal driving temp the check engine light comes on and it doesn;t like to shift from 1st to 2nd. If I park and let it cool down for an hour or so every thing seems to work ok. Ive checked the fluids every thing good.I changes the trani fluid and filter about 10,000 miles ago. Please help I am tired of pushing my car out of parking spots.