1969 Rupp 2 speed slipping-need help

i have a 1969 all original Rupp Roadster. I have issues with the transmission slipping going from 1st gear to 2nd gear. I replaced the spag bearing the first time and it was perfect. 6 months later it was doing it again so i replaced again. It is better but still have to feather the gas to get it to shift. All clutches are original. Any suggestions