here s what i can tell ya
have worked on electronic for nearly 25 years
these boxes are realy simple
if the working they work,
they only thought there after i will
give is vibration,, could cause the board inside to "lose" its grounding in it own case
most engine computors you buy for you're cars that are rebuilt, are a cause mostley to the circuit board "breaking" lose inside.
so that is why it came back working
they removed the board tested, it worked, put it back in
hey, it works, the must have fixed it!!

thus you wouldn't see anything wrong untill a certain style of "vibration " is seen by the unit
thats why it's important to use those rubber mounts, (not the only cure)