Transmission slippage

2003 Ford Focus SE Wagon, Auto, 69000 miles. Last Saturday while driving ~65mph, I felt a thump and then I lost drivetrain power in "Drive". It seemed as if trans just shifted to neutral. No warning lights came on. After stopping vehicle, I shifted it to "2" and drove it slowly home. Later Saturday, when drove it over to my garageman to leave it for him to look at on Monday, it operated OK in "Drive". On Monday, garageman looked for any diagnostic error codes in the computer and found none. Since he didn't do transmission work, I took it to over to the Ford dealer. Dealer replaced fluid & filter and said problem was due to aerated fluid. Car ran OK on Tuesday [~70 miles] but today about 5 minutes after leaving home, the transmission lost power several times again at low speed [30mph] before engaging again & then operating normally for the 30 miles I drove to work. Obviously it doesn't seem like aerated fluid was the problem. Anyone have experience with symptoms like this?