Cadillac Repair: Engine Missing, cadillac fleetwood brougham, blown head gasket

Question
My 1987 cadillac fleetwood brougham, 305 (5.0 liter) engine misses at all driving ranges. It is especially pronounced at idle, car in drive. It's as if two spark plugs are not firing. I have changed all plugs, setting to proper gap, changed the cap, rotor, and wires. I have checked the compression on all cylinders and they are all within tolerance. At higher driving speeds, you can't feel the miss, but the gas milege never lets me forget it's there.

Answer
with the engine running pull one spark plug wire off the spark plug at a time and see which cylinder(s) cause the LEAST drop in engine rpm's and this cylinder and or cylinders will be your starting point to see if they have proper ignition to them or you could have a blown head gasket between 2 cylinders if they are side by side cylinders after you perform the above test procedure.