How can I cure my Renault Laguna's intermittent automatic gearbox problem?

I own a 1999 Renault Laguna 2.0 RXE automatic which has covered 66k mailes and is immaculate. I have an occasional but increasingly frequent problem with the transmission upon which I would be grateful for some guidance.

When 'drive' is engaged, the dash panel displays an illuminated letter 'D.' Sometimes when engaged, either from rest or when travelling along, the 'D' flashes and drive is lost. It can be regained by selecting one of the lower gears, manually- i.e., 1,2, etc. When this fault is apparent and the light stops flashing, drive is regained. The car is generally well-equipped with warning lights and even a synthesised voice that tells the driver of any other faults, but not this one!

I have been told that the automatic gearbox is the Laguna's 'Achilles Heel' but I know that on some other cars, such as the Granada Scorpio Auto, vehicle management electronics can cause transmission problems that can manifest as gearbox problems.

I would be grateful for any advice , experiences or pointers on this frustrating matter...thanks.