Although I'm pretty sure your thermostat needs changing but here's the way to check it.
Your car must be cold when you do it, like leaving it overnight and the next morning it'll be stone cold.
Get 2-3 cloths, a small bucket or donga and a 10 mm ring pana.
1, Disconnect the negative battery terminal.
2, In the following picture, the thermostat is under the big pipe. Place the cloths and donga/bucket under it so that the coolant falls into it and avoid spilling on the wires.
3, Disconnect this sensor's clip. It will be towards the left.
4, (Pic from the right) Remove the 2 bolts using a 10 mm pana. Now grab onto the pipe and pull it out.
You'll immediately identify the thermostat. Take a picture of it to remember which side goes where.
Take the thermostat out carefully.
5, Use a pan, like the ones used to make tea, pour water in it so that it would submerge the thermostat completely. *Take precaution and don't 'exactly' tell your wife or mother what you're doing *
Turn on the fire, put the pan on it. Wait for the water to reach around 50?C and put the thermostat in using a wire or a chimta(tong). Make sure fire is ample to get the water boiling. Once water reaches 76-80?C it will start to open. At 90?C it should be fully open. Let it stay there for a while and then take it out. If it doesn't close then the temp drop is because of the thermostat.
Now, you said it happens 1-2 times a day so repeat step 5 a couple times if it doesn't get stuck the first time and just to be sure that it's the problem.
When you replace the thermostat with a new one, replace the gasket as well, it comes separately. Once you install the thermostat, bleed out the air by squeezing that pipe while the radiator cap is open. Pour coolant or distilled water as the level drops. Reconnect the connecter. Start your car and let it idle to see whether the temp needle doesn't go too high.
And please flush your cooling system too with the thermostat removed so that there can be more flow.
Note: If you feel hesitant in doing this you can always go to the dealership or any other trustworthy mechanic.
I hope this will help you and if I've written something wrong, other members feel free to correct me.