my engine idles low and it feels like it's about to die..(between 2nd-3rd line on the rpms). it usually happens whenever i push my car hard...not hard like i'm killing it, but i gotta get to where i'm going quick, kinda hard. anyways, lets say i'm at a stop light, my exhaust sounds like it doing some kinda of bass drum beat. the whole thing feels like it's gonna die. and if i push some gas, rpms would go up and when i let go...the needle DROPS (falls like a brick) and catches itself at the first line, just almost about to die, but doesn't. then it goes back to that drumbeat. i dunno what's up with it. distributor, wires, spark plugs were changed, compression tests taken by three different mechanics all saying it's normal. anyone have any ideas what's up here?