this goes against my experience...I disconnected my speed sensor and it still shifted in all gears except 4th....but not properly - but car has another problem..

You could try using D instead of 3rd....kidding

This is just shooting in the dark but dose your car have a over drive/Power switch? If you do have the switch dose it make any diffrence?

EDIT;

Another thought, if it has a Auto Computer (not the engine one) have you tried disconecting it and cleaning the connecter? Can't fully remember were it was but i think on my EBII it was around the centre console area..

My old EB used to do the safe mode thing when pulling out into fast moving traffic... was the Auto computer playing up. Made for fun times