Guys,
Sorry I didn't update this thread due to my work. At last I can have my bike checked and repair my steering problem.
I think I know the problem. Because a few weeks ago, I hop on my bike while it is on my garage with my foot on the floor. I tried to steer may handle and I felt that there is a delay/play (A little bit). I think I need to tighten my upper and lower nut.. I'll bring my bike in a 3s shop near our place this coming holiday and I will also try to change my front wheel bearings. I hope it would resolve my problem..

I will give update on the result.
Additional question guys, have you ever heard a "knock" on your front fork if you suddenly pass by a big hump or deep crater on the road?