When i take my foot off the accelerator, the tachometer drops to idle level (around 900rpm) on some occasions, but not on others, so I am not sure if there is anything wrong with the automatic transmission. Please advise!

The rise and fall of the tachometer depends on the speed of your car and which gear your car is engaged in. For automatic transmission cars, when the engine management unit senses that the car is slowing down, the transmission control unit changes down the gears according to the speed.

The tachometer then falls to idle when your car comes to a stop.

For a more detailed explanation, you may wish to speak to your car agent.