99 Jimmy 4.3 running really rough

Sounds like a cylinder (possilby #3) is not firing, changed the plugs, wires, oil & filter, fuel filter, flushed the cooling system. Checked the wires to see if they're sparking and they all are. Put the radio on AM and here a tick tick tick noise and can here it now sometimes when I use satellite radio. Engine light comes on when started up then flashes as I drive, only 150k miles on the motor. Had it put on a scanner by two different mechanics and they came to the same conclusion that it's cylinder #3 that is the problem and it could be a bad valve spring or stuck open exhause valve. Anyone else have this problem with the 99, 4.3, 2 door 4x4 and figure it out or narrowed it down to what it might be? One mechanic kidded that it sounded like a Hemi when it was idling. If it's the valve, is it a major project or costly?

How long has it been since you changed the cap & rotor? That could be the miss. I had that with a 97 Blazer 4.3 4x4. It is recommended to use a good quality cap & rotor. Many like OEM GM parts for this.