AUII – Auto trans problem???

[SA-AUII-MAX]

When leaving the filling station last Wednesday I did have the unpleasant surprise that there was no drive in all forward gears. It felt like being in Neutral. All sudden when I revved the car a bit (±2500 RPM) Drive kicked in with force. People looking at me if I had gone mad. Then it would drive like 'normal'. Over the weekend serviced the transmission and put a new filter and oil in. There was ± 3.5 ltrs old oil that didn't smell burned or was neither heavy contaminated. Checked the elec. wires from and to the gearbox and cleaned the connectors. Made a test ride, smooth shifting, all fine. This morning however, when the box was still cold, it had problems going from first to second gear, no drive in second. As the revs came down 'bang' got drive again and all went fine for some 30 min. When I had to turn right at an intersection, waiting for on coming traffic to pass by, I saw a gap, put foot, nothing-!!, and as the revs came down 'bang' traction, all most wrote my Fairmont off. Again people looking at me if I had gone mad.
The car has done ± 240 000 km and never had box problems before. Often I'm towing a horsebox though and fully loaded its coming close to 2500kgs towing weight in total. I'm using the heavy duty tow-pack from Ford. The gearbox has to work now and then but has never overheated or giving shifting problems.
Any clue's what's going on?