If the problem only presents itself when the car is suddenly jolted, then something is surely making intermittent contact.
As stated above, it is either owing to corrosion, or something major isn't tight. Also, the major fuses on the +ve battery terminal may have hairline cracks, and myriad other problems (maybe an ABS sensor cable is shorting).
My diagnostic technique would be thus: start the car and try to reproduce the fault. The best method might prove to be letting it idle, and hitting various parts of the engine and fuseboxes with something that won't damage anything, but will pack a punch. I jest not. Rattle everything in sight if you have inspected all exposed wiring....
Silver 08 plate 5dr 1.5dci Dynamique 106 Tech Run (re-mapped)
Red 03 plate 5dr 1.9 Dynamique dci120
Best advice for Megane owners: sell it before it bankrupts you!