My 99 5.0 cranks strong but takes 3-5 seconds to fire up. the plugs, wires, fuel filter are new. any suggestions?

Next time you start, instead of turning the key to start, just turn the key to the "on" position and leave it there for 5 seconds before turning it to "start". If the engine starts immediately, then it might mean that you are losing fuel system pressure and it is taking 3 to 5 seconds before pressure has built up enough to start each time. This would be caused by a bad check valve or a leak in the fuel system. I believe the check valve is part of the fuel pump assy.