Have to crank engine for a long time to start

After my 1997 aurora sits for a a good amount of time like 30 minutes or more. I have to crank the engine for atleast 30 seconds or so before it starts up and sometime it takes longer than that. I have already replaced the spark plugs, and fuel filter which did not seem to help any does anyone have any suggestions?

I had the same problem develope after changing plugs and wires. Some of the other threads suggested changing the fuel pump preasure regulator but before I could go out and get one I recieved my recal notice for replacing the fuel rails. What I found out was that the new fuel rails of stainless steal require a different FPPR so it was replaced as part of the recall. Now my 96 sytarts like it used to.