1999 Blazer Misfire in Cylender 6

Hello, I have a 99 blazer Ls..I'm getting a code for misfire in cylender 6. I changed plugs, cap, wires, rotor about year ago, but put in cheap plugs. So I put in ac delco last week, and still getting the code. It runs fine while driving, doesn't seem to lose any power, but when idling at light its very rough idle, and the oil pressure gauge goes down, and makes knocking sound. Is there anything else I should check or do. Any help would be greatly appreciated. Thanks in advance Mike

Have you checked the compression in that cylinder?


Could be a bad injector.