It is a 4 speed, with torque convertor lock up, on 4th, also on third, so looking at it that way it has six speeds!!
The button on the underside of the gearlever selects O/D on or off, and the other slide switch selects sport or economy.
It sounds very much like your ecu(for the gearbox) is not reading the turbine speed correctly, this is read directly from a switch on the casing that you can see from the left hand side of the gearbox(as sat in car, a large drum looking bit, with a switch with a two pin connector on it.
These play up, and can be a total arse to get out, but it sounds like this and is very common, it will also tend to make it kick between 1/2 more than normal as well.
As said though a fluid check/change should be first thing to do, also if you do towing you NEED a trans air cooler desperately as these boxes get way too hot due to no airflow around the box, unlike the us versions of fords that use this box, they have ducted air flow over and around the box.
Other common problems are fingers break on the clutch basket, later ones were modded, the pressure diaphragm splits, resulting in no go, high gear solenoid jams, the electronic change switch buggers up as well, but this is only £28, but this would violently make the box jump up and down.
tabetha