toyotaoffroad91

hey guys hopefully you all can help me again. My tranny has been doing some erratic shifting and shuttering on me. first of its a 91 pickup auto. When i go to accelerate i get a bad shutter that will rattle you teeth loose. as soon as let off the gas the shutter goes away. i also can feel the rpms going up and down while driving about 40 mph. i have replaced all the solenoids in the tranny. i have recently check them out and they are working proper with 14 ohms and are mechanicly functioning. ive noticed my rpms are a little higher than i like also. sometime it just feels like it someone that cant drive stick. do you think its my converter?? the one wierd thing is this problem doesnt constintely happen it only does it for about 20 minutes out of a 24 hours..hot or cold?!?!?! any reccomondations will be greatly appreciated. id much rather put a converter or a new lockup in that getting the tranny done becuase when it works it really shifts sweet.