1999 GMC Suburban Hard Start Cold Only

I have an odd starting problem - it takes 3-5 attempts to start my Suburban in the morning (more on colder days), runs rough for about 5 seconds then smooths right out. However, it starts just fine the rest of the day, even if its cold. It appears to be starving, as adding starting fluid lets it fire right up the first time. Once it runs there is no lack of power, rough idle, poor acceleration - it runs great. At no time does the Check Engine light come on indicating a code has been thrown. Also, my fuel pump was replaced about 11 months ago.

Things I have done: new plugs, wires, rotor, cap, air cleaner, O2 sensors. Pulled upper intake manifold (plenum), replaced CSFI fuel regulator, pressurized system and checked for injector tube or other leaks - all good.

My manual and everything I read online says that when checking fuel pump pressure at the fuel rail, you should have 60-66 psi when the pump is running, cranking should be 50-57 psi or it won't start, running should be a minimum of 55, and after the car is off it should hold 45 or better for 5 minutes. If it drops to 0 within 15 seconds, you have a leak under the plenum or in the fuel line. If it doesn't have at least 50psi, the pump is bad.

Here's the thing - my symptoms are right in the middle of all this information. When the pump is running, I only get 54-58 psi. Cranking, I only get 44 psi (which according to everything means it won't start but mine does), and then while running I DO get the recommended minimum of 55 psi. After it is off, it doesn't drop to zero within 15 seconds (indicating a leak), but it doesn't hold above 45 psi for 5 minutes, either. I get: 48 psi after 1 minute, 40 after 2 min, 36 after 3 min, 33 after 4 min. And 32 psi after 5 minutes.

Since the fuel pump is fairly new, I hate to replace it again just to find the issue was with something else. Do you have any ideas of what else might cause these peculiar cold hard start symptoms?

Thanks for your help!