1998 Mazda 323 rough idle

Engine Performance problem
1998 Mazda 323 Manual 170000 miles

Im having problems with my car's idle speed. Most of the time when I start the engine the idle speed moves up to 1.5 - 2.0 then goes back to 800 after a few minutes. This also happens while on the road when I slow down and shift to neutral, the idle increases again and takes time before it goes back to normal. But when I release the clutch when in gear, the idle goes back to normal and sometimes even go lower. Another problem is that when slowing down to stop from a high rev, the engine stutters and the rpm goes below up to 100 and sometimes the engine dies. What could be causing this?