Transmission slipping out gear

It's a 1995 A6 Quattro - auto transmission. The problem being experienced is when driving the car and the car is about to shift from 3rd to 4th, it appears to shift out of gear for about 2-3 seconds before going into 4th. So what the driver sees and feels is the RPM climb brief and coasting before the car shifts into 4th.

Anyone else experiencing this? Any suggestion? Is this major or minor?