Multiple cylinder misfire

So my car has had a poor lack of power for a while now, struggling to go over forty mph to be exact.
The engine management light came on, I ran a diagnostics test and it said multiple cylinder misfire, I have replaced the spark plugs and the coil pack, but the car still idles at 500 rpm and cuts out after two minutes or so if you do not rev the engine a little.

I want to know if anyone knows exactly what it may be because I do not just want to keep throwing money at it if it will not fix anything.