I tend to agree with the Loose connector or bad ground type of problem, but if that isn't it, there are other less likely possibilities:
Too many keys on your key chain? Sometimes people hang lots of keys and things on their key chain and it messes up the ignition switch. When they hit a big bump it would move things around in there.
Probably not it, Just an idea to try.
Also, an old battery could have problems when bounced hard enough (again, not likely, but something to think about). Maybe the battery is shorting to the hood if it's too tall or the hood padding is missing???
My 2 cents worth