Just finished a trip driving my 1973 ES and came down with an overdrive problem. While driving 68 MPH at 2800 RPM in overdrive, I noticed the tach going to 3500 and back down to 2800. I shut down the radio and could hear the motor over reveing in time to this. I could use the switch to go back and forth between 4th & OD, but it seems like at times it was only partly into OD. I parked it and checked the oil level and it was 1/2 a qt. down. I topped it off and drove it another 175 miles. It worked well on the freeway above 65MPH but was very intermittenty below 50 MPH. Any ideas on where to start on this one???? Thanks for reading this post.