Motor trouble with 1999 Kawasaki Vulcan 1500?

I have a 1999 Kawasaki vulcan classic 1500 street bike. I can get the bike to idle when its cold, but after it gets warm it starts to idle really rough unless i give it gas but after i let off the throtle it sputters out and then dies. I have adjusted the jetting, checked all the plugs(4) for fouling. Also when the motor starts to die the sound of the exhaust changes to a more"hollow"sound. I dont realyy know how else to explain it, any help would be appreciated.