I recently inherited a 2001 Taurus with 61K on it, a CA. car that's in nice shape overall. The other day I was on the expressway and let off the accelerator for a few moments and coasted. I wasn't using cruise control, was coasting about 45 mph with the air on. When I hit the accelerator again, the engine rpm went up and the trans felt like it was slipping until it engaged with a hard thump. I was concerned and tried it again a few minutes later to have it repeat, but without as much slip. On the third try, the trans acted normally. Other than this, the trans is shifting normally with no slip. Anyone have an idea of what's causing this? The fluid level is correct, and it's clean.