Alexander Valde Sylte

Hi, I have a 1997 A6 2,5 TDI with a very irritating problem. The speedometer and all the other instruments randomly stops woring and show no sign of working. When this happens the light for low coolingfluid starts rapidly blinking. If I stop the car and take out the key everything works flawlessly for a random amount of time(anything between 3 mins to 3 days!). I anyone has any tips or know whats wrong, please help me!