Does it take a while (up to but no greater than 5 seconds) to engage reverse?

Does it make any noise while (as you put it) slipping?

Are you checking the oil level with the engine hot, running and in P or N?

How long has it been since you've had the pan off and changed the filter? Were the bands adjusted properly or just 'nipped up'?

Does the box shudder as you try and drive away or as the drive takes up (re-engaging gear)? This is a bit of a biggy as it can indicate if the person who built the box built it to suit a V8 or 6 (wrong spring in the band servo).

What you've mentioned could be something simple (blocked filter or low level or even just the wrong type of fluid) or a much larger issue (shagged oil pump or second gear clutch pack stuck on) or similar.

I'd suggest getting it a tranny shop and show them exactly what it is doing.