[Opelotus]

There are seven main ground points shown in the workshop manual;

battery ground strap
two clusters of grounds above the central electric board (passenger footwell)
engine ground (passenger side underneath car)
ground on front left crossmember (near headlight drive tube)
ground on front right crossmember (also near headlight drive tube)
ground under instrument panel/steering column
ground in right rear corner of trunk, under trim panel

does it only happen on bumpy roads?

WSM states that the central warning computer is supplied voltage via a specific terminal when starting the engine which cancels the bulb check function. A faulty alternator can cause the warning lights to flash all at once. If some flash individually then it could be loose/corroded connections for those particular circuits as well.