The 700R4 in my '89 has been having an intermittent problem. Under hard accelleration when in OD over 55 mph, it suddenly feels like it comes out of gear and the engine revs free. Sometimes it will vibrate bad and feel like it's trying to shift back in. Other times, you can let up on the accelerator and it will go back into gear. Otherwise, it shifts and locks up just fine. The fluid looks good and I've tried setting the tv cable with no improvement. Any ideas?