O2 wont make the car run noticably different . youll get worse MPG but its not a power or drivability issue.

Yes it can still be the dizzy even if RPMs arent affected. Usually, the car will run better cold , as temps rise under the hood, the ignitor contacts short out and the car will run crappy in the higher speeds and RPMs.

i would change the dizzy to eliminate that as a possibility, then check your timing and compression/leakdown after that