Snapper 285Z Running Rough?

My Snapper 285z seems lately to run strong when the blade is engaged and I'm mowing, but when I disengage the blades it runs very rough and sometimes dies. It then takes a good while before I can restart it. Once I restart it, it will then run rough again until I engage the blades then it again will run good until I disengage the blades again then it runs very rough again.