Eventually I think I have cured the problem. Thanks for all the invaluable advices offerred by all of you.
I have been working very closely with my mechanic. I have seen for myself every single piece of component/part that has been replaced. I have come to the conclusion that the problem was due to two parts.
The first part is that (this is the latest discovery) my front right brake disc rotor was distorted/deflected. Therefore when it was heated up it curved, thereby jamming the brake pad and caused the wheel to shake.
The second part is that the left upper arm of the (left) wheel which joined the lower carriage was damaged. This was diagnosed and had been fixed earlier.
Now my conclusion is that the first part was the main cause of the shaking, and with the second part, the shaking was simply magnified.
The breakthrough came from my diagnosis of ruling out the engine/fuel/carburettor problem. When the shake occurred, I stepped down the clutch and brake pedals and I could feel the same type of vibration came from them (and the engine worked perfectly). So, we have zeroed into the clutch and the brake system, with the latter at a higher priority. Moreover, I gradually believed that the problem was "driving time dependent" rather than "speed dependent" - or to be exact, "heat dependent" which explained the heating up of the distorted rotor. The "come and go" phenomenon happened probably because the rotor was heated up to different extends at different conditions, etc.
This is a good learning process, well at least for me. Again, I would like to thank all of you who have offerred help. I do hope (touch wood, who knows) I have genuinely conquered the problem and the next time I put up a post would be offerring help/advice.