1999 Blazer Misfire in Cylender 6?

I have a 99 blazer..motor is out of 00 astro van..but is 4.3 L...i was getting code for misfire in cylender 6. about a year ago I changed plugs, cap, rotor, and wires. I tried changing plugs to AC delco's like called for, but still getting code. It has a very rough idle when sitting at light, and oil pressure drops down while sitting. But when driving it seems to run fine, with no loss of power. any suggestions on what else this could be. Thanks