Take a look at the trans cooler lines which run along both sides of the oil pan. They are attached to the pan with clamps, with a rubber gasket.
This gasket hardens and causes the line to rub against the clamp, thus creating a hole in the line.
This happened to my 81 300TDT.
I had misc. leaks underneath, but could never track them because the fluid would just drip slowly at first, and run all down the underside while driving so it looked like my entire frame was dripping.
Then, one day out of nowhere, I had this puddle under my car. No slipping or other noticeable trans problems for a while.
Then blammo! Slipping up the wazoo.
So, replaced with a section of rubber trans line and all is well.
This may be a long shot, but at least worth a look.