catboat

I have a '99 Forester, automatic transmission, 4 cylinder, with 60,000 miles on it. While in "drive," and coming to a complete stop (ie stop sign, light), I will sometimes experience transmission slip. I press the accelerator down, and the engine revs-then the transmission engages. It doesn't happen all the time. It doesn't seem to matter if the car if hot or cold. Fluid level is good-perhaps just a little high (when hot, and engine running). Had the same problem last summer. Took it to a Subaru dealer, and they did a tranny flush. I asked if they changed the tranny filter, and they said it doesn't have one. (Is this correct? Seems odd). That was at 50,000 miles. It seemed to solve the problem, up to a couple of weeks ago. I bought the car used last summer with ~ 47,000 miles on it. It has not been used to tow. Any thoughts on what can be looked at? I don't have any transmission slipping in any other gear, just going from dead stop to first. Thanks in advance.