94 buick lesabre. 119,000 miles. happened 2 times on different occasions. only first part of day, first time started that day, at about 50 degrees. Start engine, shift into either reverse or drive, and vehicle wont move. push pedal until engine revs high and car slowly begins to move. after about 30 feet i stop, shift through all the gears and they each catch like normal, and vehicle drives fine. no hard, soft, delayed or otherwise noticeable shifting problems while driving, stopping and hand shifting to different gears, or turning on and off and repeating process. vehicle has not been abused and has been well maintained. Transmission flush was completed about 250 miles ago. Fluid levels are good and have been properly examined by following directions in manual. Thank you for your time.