At last - speed bumps have their uses! Can't see where they are coming from with the ignition relay switch idea (though they may be right!). Sounds to me like either a loose electrical connection or something physically stopping the fan from turning which is dislodged when you go over a bump and once running has enough momentum to overcome this resistance. Try and access the actual fan and see if it will spin freely by hand - if it will then it's probably an electrical fault, if it won't spin freely see what's stopping it!
Richard
December 2008