Transmission pops out of sixth gear

My car has a six speed gearbox. When gearbox is shifted into sixth gear, with any engine load on it, it will pop out of sixth gear only. Replaced gear shift linkage with new Toyota parts. Also replaced oil. Did not cure problem. Clutch was replaced about 5,000 miles ago. Car purchased about 30,000 miles ago, I do not race, but I do not baby it either. I do not know the history. I changed all fluids when I bought it.