Severe engine mis/stumble, much worse when cold

[toms68cst]

Hi folks,

I’ve been having some severe stumbling/mis on my 68 I can’t figure out.

When cold, or even before it reaches operating temperature on a warmish day, it will hardly rev up under load. I can accelerate out of first but in second or third gear it will stumble severely. It's better when warmed up but running on the highway or letting it rev hard under load, it will still stumble severely.

It's been very cold blooded in recent years exhibiting the same stumble but previously only when it's very cold outside.

Things I’ve done recently:

Replaced spark plugs (The old ones looked good), Replaced fuel filter at the carb. The sending unit is newish so I think the sock filter there is OK.

The choke pull-off seems to work ok and the secondaries will kick in when it's at operating temp.

My mechanic checked the timing and the fuel mixture recently.

It’s the original 327/q-jet. The q-jet was rebuilt about 4-5 years ago by my mechanic. It has a 10yo stock-look Pertronix distributer. (I have a new ignition coil coming.) My mechanic said the distributer cap looked OK.

I can’t say for sure if it’s the carburetor/fuel or ignition at this point. Any ideas for troubleshooting?

TIA, Tom