Running Terrible..

Its a 1992 300ZX N/A, 2DR.

Well..Two days ago i cleaned the K&N Air filters, and changed all plugs, since then the car doesn't wanna keep a steady idle, miss fires more then it did, doesn't feel like its putting out the power it should be putting out when pressing the gas while driving, and sometimes while driving the car just dies out, and i gotta turn the key or put it in gear and let out the clutch for it to turn it back on..what could be causing this? Does any1 have any related problems?

Earlier this morning i took out the coils, and checked all the spark plugs, 3 of them were really really black..like very bad, something that shouldn't be done to a plug within a few miles of driving. And they were a lot worse then the ones that were in the car when i did the change 2 days ago..The other 3 were dirty, but not as bad as the other 3. Also when you press the throttle u can hear like a "suck" of air somewhere..looked at all vacuum hoses, but don't see anything missing or disconnected.