My car is 1984 W123 2.0 liter gasoline, 4 cylinder running on carburettor. Recently I have experienced some "shaking" problem. Details as follows:

(1) It is shaking, not vibrating. The shaking comes from the engine, not the type of vibration that comes from tyres/wheels.

(2) It is come-and-go. Sometimes I cruised for 20 km at 120 kph, no problem. Sometimes I drove at 90 kph for 10 km, the problem kicked in.

(3) When the problem happens, the whole steering wheel and gear level are shaking. I have to naturally slow the car down to below 80 kph. Then it is OK. But pressing accelorator again will tricker the problem again.

(4) I have checked and repaired all electric wires, spark plugs, and even the air filter. No help.

My workshop seems to be of no clue too.

Any advice would be appreciated.