1992 Toyota Tercel transmission problems

Our Toyota is having weird problems in the transmission. We love our little car especially now with the price of gas. But When we start it, it jerks real :banghead: hard and then it works fine till it gets hot. I mean it shift so nice and smooth for about 15 minutes until it warms up and then it grinds and you can't get it into gear. Can anyone help with this problem?