transmission question

OK I know that not many people have or know much about the 85 toyota automatic transmissions, but i have to ask cause I'm beyond confused. I changed my tranny fluid cause my truck was sometimes not down shifting when i took corners and sometimes going straight. So after i changed the transmission fluid the tranny was fine for a while and then today it did it again..im so lost is my transmission going out or is there something else i should check for. Any input would help thanks guys