98 Tahoe 4DR 2WD 5.7 won't start after sitting...Please Help

Hello all...I am new to the forum and this is my first post and I am in dire need of help...I am working on a customers 98 Tahoe with the 5.7L vortec...The problem is that after sitting for, on average, 2 days it won't crank...It will turnover but it just won't hit a lick....But after being run for even a short period of time it will crank fine as long as it hasn't been sitting cut off for too long...Now here's the catch...If you hook the battery charger to it, it will fire right up...So to make it clear...It will always turn over but after it has been sitting to long it just won't fire up unless the charger is hooked to it...I've worked on a lot of vehicles but never seen or heard of anything like this...I've replaced the fuel pump relay but after letting it sit for two days it is the same problem...Fuel pump is running but I noticed after it sits the fuel pressure isn't building up to the required 55-60 lbs. of pressure when I hooked the gauge to it...Don't know what to do and I hate to run up a bill trying to pinpoint the problem...Open to any ideas...Thanks

ignition on engine off you need 60-65 psi. the pressure should hold. if the pressure rises and then falls off the pump is bad and the fuel system may have a leak. like injectors/regulator etc.

then you place the gage on the fuel filter fuel pump output and see if the pressure still is too low and falls off. then its the pump.

when the engine is cold you need more fuel pressure/volume !

any low pressure issues will show on a cold start.