I have no idea about your problem but i also faced similar problem with my toyota car. When i brought ( second hand ) it, i noticed that there is an abnormal heat after parked. but the heat indicator did not light up while driving. after few months later when i was on my way to n- eliya, RED heat indicator lighted up.
I changed coolent , radiator. but the problem was there.
later i discovered that there was a small gasket leak in the engine. While driving on flat areas it was not a issue to the car. but when it get load, it could not bear the heat.
the best thing is compare it with another Wagon R . Also start the car and open the radiator cap and check if it gives any bubbles.
modern vehicles does not come with heat meters. that is a problem to check this types of problems .. i personally do not like bulb indicators .