Problems starting! Please help!!!!!!!!!!1

99 with 5.3 problems cranking. Cranks up good. When you shut it off and wait 5 to 10 minutes cranks back up fine. If wait hour or more it dont want to crank. It will spin over for about 5 to 10 seconds befor it cranks. If it sits over night and you crank it, it cranks up fine.
Use to have problems with idling but that problem has disappeared. Have noticed since it has started getting cold that untill it warms up it runs very poorly, spitting, sputtering, and backfires. Once it warms up it runs with no problems.
Want to fix the problem badly. Went to Auto Zone had it put on their computer and it said it was a firing problem. Changed plugs and wires but nothing else.

If it's a firing problem, I would suspect your coils. However, the way you described the starting problem, it sounds like fuel filter and/or fuel pump

My friends '99 did that. We suspected the fuel pressure regulator on the driver's side fuel rail. Pulled the vacuum line and fuel dripped out. He replaced the regulator. That solved his problems and his fuel mileage increased.

Are your battery cables tight, or have you had it tested? If it doesn't want to crank after it 's warm, you may want to look there first. (By "Cranks", I'm assuming you mean it turns over, but doesn't start. There's a big difference between "Cranks fine", and "STARTS fine")

(Recent cold weather also can cause alot of IAC problems, I've been starting to see quite a few of those pop up in my shop lately. You may need to clean it, and the throttle body as well. I believe GM has a TSB about this, Hard start, poor idle, etc.)

100% fuel pressure reglator

Fuel Regulator