I have a 1999 Jeep Cherokee that will not start. Sometimes, I turn the key and it starts just fine, other times it will not fully start. I turn the key and the engine will groan but not start, I try lightly stepping on the gas at the same time and nothing will happen. I do hear the fuel pump turn on, it has had a new distributor, and ecu installed. The starter does not click, sounds fine. I took it to the jeep dealership and the ran a diagnostic, no codes were in the computer and of course it started every time for them. Any other suggestions or thoughts would be greatly appreciated!

I wonder what that means when they said they "ran a diagnostic"...
If they checked the system for diagnostic trouble codes (DTCs), they may have not done a thorough test of the vehicle's starting and charging system.
You need to find out.
The battery should be fully charged and tested to make certain it doesn't have any bad cells (which can mask themselves and work fine intermittently). Both ends of both battery cables need to be clean and tight. Ater that, when the fault is present, a voltage drop test to see exactly where the fault is.