Thanks for all the advice above. But, the problem still there.
I have just changed the carburettor to a refurbished one. Everything seems working fine but the shaking problem is still there.
I have discovered one thing though. While I was driving and the shake came on (still "come and go"), I pressed the clutch and pressed the accelorator pedal, the engine rev went up smoothly. So, the shaking did not seem to come from the engine itself - hence it did not seem to be the (occassional) fuel starving problem.
When I pressed the brake pedal (while pressing also the clutch), I could feel the shaking from both the clutch and brake pedals quite severely. The shaking is like horse gallopping.
I have re-checked all the wires and spark plugs and confirmed that they were all right.
So, anymore insights? Someone mentioned "driveshaft" problem earlier. What is it and how to diagnose it? I still don't understand why the problem is "come and go".