Ford Repair: wont start occasionally, mercury sable, ford product

Question
I have a 1999 mercury sable which is a Ford product.  Sometimes the car won't start. It turns over fine but won't fire.  Took it into the shop, the computer is clear and the condition didn't happen for them (of course) so I am trying to get some other ideas. Since it will turn over, I figure it is either a fuel or a spark issue.  The shop said the fuel filter is fine also.  I was thinking the fuel pump may be a possibility.  Any ideas I could try?
Thanks,
Ross

Answer
couple of questions. first i need to know what engine is in the vehicle. second, when the vehicle wont start is there any chance you notice if the theft light on the dash board is flashing ? when it doesnt start, is there any type of repetition you seem to do before it does finally start ?