How can i fix my Volkswagen Sharan VR6 1998 automatic transmission?

The automatic transmission gear of my Volkswagen Sharan VR6 1998 behaved funny last week. The drive gear became very weak and later stopped moving the car. After I serviced it by changing the filter and atf, the car started moving but very weak. It was then found that the brainbox was malfunctioning. This was changed, but still seem to have no effect on the car's beahaviour. please help me.