A little more information may be helpful in diagnosing. Such as, does it start like normal when it does start? Does it smoke at all and what color is the smoke if so? Does it act like its starving for fuel when it shuts off or does it shut off quickly? What does it do when you try to start it immediately after it dies? Does it just click? Do you get lights on the dash? Does it turn over but won't start?
With extra info I'm sure you can be pointed in the right direction!
It does crank normally and runs for approx 30 seconds and it shuts down slowly as though no fuel. It will not crsank at all until it sits for several hours, the motor turnsjust fine and the lights come on. It just wont crank for several hours and then it does the same thing again.
I am thinking like Tooljunkie. Check to see if you have a good flow of fuel up to the injector pump. If the filter is running slow you may only get 30 seconds of run time out of fuel in the line. Letting it set allows fuel to slowly fill the line again. Remember we are just guessing at this point.