I have a 1993 Corolla Dx 180k miles with a automatic 4 speed transmission and a 1.8L engine. I recently changed the transmission fluid and filter. Approximately 4 qts of dexron 4 ATF. Before this the transmission, after awhile, would not automatically shift. Had to put in 1st. However when put into 2nd it was in 3rd. No difference between 2nd and 3rd (D) positions. After I changed the fluid it shifted just fine for a couple of days and now it's doing the same thing. The fluid before the change was dark, but did not appear to be burnt. After the change fluid is a little dark, but still pretty pink. The car will move in D position, just slower than when I put in the 1st position. The car seems to shift ok until it warms up. A friend suggested that I take the cooler lines off and flush the cooler. I asked about flushing the transmission, he suggested that I don't. What do you think.