Someone with diagnostic Knowhow...PLEASE

hook-up a fuel pressure gage to the rail and see what happens when you shut it off, see if it drops to 0 instantly...sounds like it not holding pressure....hence the starting issue... you can try to prime it 3 times or so then hit it to start....
didnt see anything in your sig so assuming its stock the one and has some miles on it, that might be your problem???

Like Rio said, first thing to do is hook up a fuel pressure gauge. The permanent type would be best, that way you can pop the hood at any given time and check it. I'm a tightwad so I made my own adaptor, but www.50resto.com has a gauge kit for $39.95 p/n KP-1441. The fuel rail on the passenger side of the motor towards the front has a schrader valve (looks like a tire valve) and that's what you hook on to. You need to hook up a gauge to the valve and see 1) what pressure you get (with and without the vacuum line hooked up to the pressure regulator) and 2) how long does it take for the pressure to drop off after you shut the motor off (bleed down).

Start with checking your pressure and be systematic/organized/logical as you trouble shoot. Also, just use common sense and follow your gut! I had this same problem with hard starting every once in a while. Hot, cold, didn't matter. I'd jump in and it would take 5-8 attempts until it would catch. Sounded ignorant to someone standing nearby and hearing you crank and crank! 1/2 way through my "trouble shooting" I thought ignition switch. PSI readings were good but everyone said no way on electrical, had to be fuel pump. After replacing a half dozen parts (including the fuel pump which means dropping the tank and dealing with the PITA fuel fittings that won't release) guess what. A $14.00 ignition switch solved the problem.....

Just take it a step at a time and think as you go.