transmission slipping, not engaging

i have a 69 super six 225 with a four speed auto
last week after driving for about 8 minutes the trans started racing
in 3rd gear rpms would go up to 3000 at 30-35 mph
later at a full stop light turned green and i pushed on the accelerator, engine reved in drive, and a second time, but the third time it reved it slowly engaged a started to drive forward real slowly
couldnt push it over forty and rpms were 3000 at 30 mph in drive?
any ideas?
its not the kickdown cable