[Horneck]

I have not made any progress finding out what my cars problem is. Have removed the intercooler and have checked for loose hoses or faulty o-rings. Unfortunately nothing found.
I have a 40 year old CO tester that I am not sure works correctly and have used it to measure the CO at idle. It showed 6%. Not sure that I trust the device though. I bought a AF ratio gauge with an oxygen sensor but unfortunately it shows only 0. Somebody told me I have to calibrate it first which I will try in the next few days.
As my car is running fairly rich the sparks were very black last time I checked and cleaned them. Is it possible the fail under load? Should I change them? What do you think about changing the fuel filter as I have used seafoam engine restoration in the car.

The symptoms of the car are currently as follows:

Car starts immediately and idles great.

If I drive the cold car immediately I can rev it up easily to 5-6k but I don’t want to push a cold engine. After approx 5 min driving my rev problem kicks in again. The car starts now to misfire around 2500-3000rpm refusing to rev any higher. If I accelerate the car with only 25% throttle I can rev it to 4000 but the second I hit the throttle harder I get immediate misfire.
If I rev the warm car at idle I can easily rev the car to 6000 but this is without load. Am I right to assume ignition is not the issue and I don't have to check coil, CDI or distributor?
Appreciate your thoughts.