Auto gearbox clonks!

Car:
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Seat Arosa automatic 1.4L (VW Lupo with Spanish accent!). R reg.

Symptoms:
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Low speed:
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* on cold, start-up, gear change clonks/thumps, this clears up with a couple of miles.
* sometimes, with a loud clonk, there appears to be a change of gear?Upwards (sorry no good detail as quite rare and happens, as far as I can tell, at random.)

High speed:
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Motorway, around 70 mph: pootling along happily when, randomly, revs climb rapidly from about 3000 to the region of 4000. Then, a few seconds later, all is back to normal. Sometiems, this is accompanied by a clonk. This happens in wet or dry weather, the gearbox has been checked and levels are ok and there is no grot in the filter. This used to happen two years ago, then the gearbox was serviced and all was well until a couple of weeks ago. Lastly, the enine management system said that a switch was brokewn that tells the EMS that the brakes are being applied. But this never happens when braking, so probably irrelevant.
I have jsut done 600 miles in the car and there appears to be no permanent damage, as far as I can tell.

Any ideas? Is the gearbox liable to just seize up on me when going fast (very embarassing)?

Many tahnks,

pete