Gearbox Problem

[samsx5]

Hi,
I have got a 2000 4.4i sport with an auto gearbox. For a few weeks the gearbox has been playing up. Mostly when cold it wont change gear when its suppose to. I can switch to sport mode and shift freely through all gears without a problem and when I return to auto it works fine and its forgoten about but then a few days later the problem re-occurs. I have just nipped out to the shops and it will not change via the autobox, but the manual works fine. I have stopped driving her anyway as I am replacing the rear ABS speed sensor today (hope this has nothing to do with it). Thanks