My mechanic is having trouble finding what is wrong with my E320.

The fault is that the car loses power suddenly and the speed drops to 20mph.

By selecting Park on the auto box and then returning to drive temporarily fixes the fault.

My mechanic has cleared the fault codes but the gearbox showed no faults.

Has anyone else come across this as my mechanic seems to be having problems sorting this.

The car has suffered a gear selection problem in the past but this was fixed by the Main dealer by changing a component in the gearbox.

Please can you help as I love the car but this problem is driving me to getting rid of it.