Hyundai Coupe 1997. Cuts out when driving over bumps but only in the mornings.

There is no option for Hyundai Coupe 1997 in the selection box so please
ignore selected car. So i leave for work at 6:30am and once im driving over
bumpy roads the speedometer starts to flicker up and down causing the car
to jolt. while doing this the check engine light and seat belt light illuminate
just as though starting the car. Sometimes the car will be ok and it'll stop but
other times the car completely dies. Upon trying to restart the car i get no
lights on the dash at all and the car wont restart. After waiting a while the car
will restart again. I took a video of the car acting up (On a private road near
work) and showed it to an auto electrician. The auto electrician couldn't get
the car to act up as they didn't open until later and it seems to never happen
when its warm. They had it plugged in to 2 diagnostic machines with no error
codes displaying. Could the car be getting moisture to some electrics
somewhere or could this be something else? Thanks in advance.