Thanks for the advice.
I want to add that the shaking is really very rigorous, as if the engine wants to die.
I have also changed the spark plugs, thinking that could be the source of the problem. No help. After three days of use, my workhop and I inspected the (new) spark plugs and found that they are "blackened", only after three days of usage. It looked like it was due to fuel flooding.
So, the question is, could the "shaking" be due to a shortage of fuel, but only at high speed cruising after some time, while at normal speed and driving time of the car there is a oversupply of fuel?
We suspect that there are two parts to the problem:
(1) The over-supply of fuel at low speed could be due to a malfunction of the carburettor; while
(2) The under-supply of fuel at high speed could be due to a malfunction of the AC fuel pump.
Are we on the right track? Any more insights?