1999 Mitsubishi Montero Idle drops from 1K to zero when war

I recently replaced the battery on my SUV. After that, the truck would stall upon stopping, idling, or coasting. my husband noticed that it doesnt stall on initial start up. It will stall when the engine is warm. then the idle will be steady on 1K then drop to zero and stall. We can be at a red light or even coating into the driveway. Any ideas? Thanks, Angela