A week after returning from the June W-Body meet, my transmission started shifting erratically between 2nd and 3rd gears.  From a dead stop, it would shift out of 1st normally, stay in 2nd for a split second, then shift to 3rd, causing my engine RPMs to go down until I got up to speed.  When I got my speed up, it would shift fine between 3rd and 4th.  I checked the fluid level and it was just under that "add" line.  I replaced the filter and fluid and it seemed to resolve the issue...until yeste