'05 4.3 misfire, stumble

[TheManInBlack]

Hello All,

2005 GMC Sierra 4.3 4x4 regular cab short box

Recently my check engine light started coming on while driving. I went and had it scanned and got coolant temp too low and random misfire. I replaced the thermostat and now it heats up properly. I also replaced the spark plugs and the spark plug wires (the plugs were extremely bad, the worst I've ever seen). It starts much better now.

The current symptoms are as follows when cold in the morning (10-20 degrees F) it stumbles and misfires terribly but runs good if I kick it down to wide open. Check engine light is on all the time and blinks randomly while driving.

After the engine heats up when I'm on the interstate it seems to be cutting in and out slightly as I can feel it surging. Once again if I kick it down it seems to run great.

Also around the same time it seemed to develop a vibration (not sure if this is related or not). it vibrates through the steering wheel and gas pedal mostly when at part throttle 20mph - 60mph, less when off the gas and seems gone when at wide open.

From what I have read on here I am starting to lean towards cat converter problems/oxygen sensor.

What do you all think?