Electrical problem
2003 Renault Laguna 4 cyl Two Wheel Drive Automatic 76000 miles

Most of the time, but not 100%, soon after starting the car, the red warning light comes on, with the instruction to stop immediately. However, after 1 or 2 minutes the problem goes away, and doesn't recur all the while the engine is running. However, stopping and restarting mostly produces exactly the same symptom.
A number of garages have looked at the problem - they've cleaned any contacts they can think of, and the last garage said it was a battery problem, so I bought a new battery, but this hasn't solved it. It seems to me that it must be caused by something that only runs soon after starting, and then isn't used again. I thought it might be the starter motor, but it doesn't come on immediately after start, just a few seconds afterwards, so I assume it can't be that.
Any ideas?