I,m pretty sure they run an external voltage regulator, mounted on the side in the engine bay, its been a while now so I'm not sure if the original had points inside which pit, stick and so on. Get yourself a fully sealed electronic one from the wreckers, as chances are someone has updated to cure symptoms like you are having, even one from another Toyota model eg Corona, will do, If yours is the original it will have a big black top on it, No more problems This is what you need;
Buy Jaylec Voltage Reg 12V Denso Ext 026000-4050/4051/4070 For Toyota In589 15-867 Online | Rolan Australia