Coolant temperature gauge does not remain constant after reaching
the normal. It increases during idling, stopping and slow speeding.
It shows decrease only when the car starts reaching at a speed.
The lower and upper radiator hoses are just warm and there is no loss of
coolant.

Please help me out in solving the problem.

As long as it doesn't overheat while idling there is nothing wrong.

+1. This is normal. The variable is the speed of air running through the radiator. At idle, the speed is zero and so the engine gets warm untill the fan kicks on. On the highway the speed is greater so the engine is kept cool.

As long as the needle doesnt hit red and the fan turns on at 3/4 per the gauge...your fine :)

my 99 s/w does the same thing i agree this is normal.