1995 Chevy Blazer missing low idel

My chevy blazer misses in park and when im at a red light it almost stalls idel goes from about 900 to 500 or less up and down. Whem im giveing gas runs good, sometimes I think I feel a surge when changeing gears but idel and in drive stopped misses really bad. Have replaced o2 sencer egr valve and one other