Have a 2001 dodge ram 5.9/v8 1500. cranks but wont start. if i leave the battery disconnected for a bout 15-30 mins it starts up. but then dies after i let it idle

What have you done for diagnostics so far? Is the engine light on? Did the ECM store any codes? Besides the current symptoms, was there anything else you can add to this issue ( like, did it happen out of nowhere, happens when its cold, hot, wet). I need more info, several things can cause a no start. But in your discription it sounds like a vacuum leak, I.A.C.valve, TPS sensor, or maybe map or maf sensor. But it is crazy to go further until you can run some diagnostic test and narrow down, the cause of your problem. I can say that it sounds like it is related to the ECM and when you disconnect the battery, it is putting the ECM into default, and as the vehical is running it senses the fault and causes the engine to die. Try to provide some engine codes if possible.