Auto gearbox fault on A4 2.5TDi

I would be very grateful for some help identify (and solve!) a gearbox fault for me.

My car is an automatic A4 2.5TDI (1998) with 130,000 miles on the clock. Intermittently the gearbox warning light will illuminate and the car will then run in 4th gear only with what feels like limited power. Switching the ignition off and then back on will clear the problem.

When the warning light is not illuminated the gearbox is smooth, quiet and fully functional in both auto mode and when selecting gears manually.