Change the transmission fluid and filter and put the right fluid in. That shudder is a chatter from the torque converter locking up in third gear above about 40 mph. The wrong type of fluid causes that.
You can prove this is the cause by driving at a steady speed above 40 mph with the engine warmed up. Hold the accelerator pedal perfectly steady, then tap the brake pedal. That will cause the torque converter to unlock and relock a few seconds later. You'll feel the same shudder each time you do that. You may hear the engine speed pick up a little when you tap the brake pedal. If you have a tach, you'll see the engine speed increase 200 rpm, then go back down.
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Monday, September 1st, 2014 AT 7:50 PM