Sigh - problem still persists.
In addition to all the work done mentioned earlier, I have in the last couple of days (1) replaced the fuel pipe leading to the carburettor, (2) replaced upper arms and the bushes under the carriage (3) done another round of wheel balancing.
What I am going to do next is to replace the engine mounting, gear bushes, etc.
Could it be due to the engine valve problem, which means one (or more) cylinder is leaking? And, why the problem does not happen all the time?