99 boxster automatic transmission slipping

[NewArt]

Hi Tom, Welcome to the forum.
You say you changed the fluid and filter. Do you mean YOU changed it? If so, that's great.
When I changed my fluid/filter/wiring harness on my '01 tip, I had to check/top up the fluid 3 times to get the right level. It's not a hard job to top it up but you need to get the temperature right (use an infra-red thermometer for this) and add fluid until it backflows out of the hole. When the fluid is low, the transmission will slip. This may not be your problem but this was my experience. Good luck.