I don't know your expertise on engines, so please forgive me if my reply doesn't reflect your level of knowledge.
Do you really mean you can feel resistance on the pedal, or feel that the pedal doesn't return immediately when you remove foot pressure, which could be caused by the linkage rubbing against something.
Or do you mean the rpm just doesn't fall off as quickly as it should upon shifting, so you assume the linkage might be sticking?
Or do you mean the car "stumbles" when you feed it gas right after shifting? What about if you just press down firmly on the pedal (without shifting) while cruising in second or third gear?
If the first, then it could be a bad motor mount allowing the engine to roll too much on clutch engagement and bind the linkage, or an air cleaner that was put on slightly off it's correct orientation/position and it's binding the carb linkage (throttle or choke), or bad throttle linkage bushings (or just a need for linkage lubrication), or even the gas pedal hanging up on the rug or floor mat.
If the second, it could be the clutch going bad and slipping on initial engagement.
If the third (hesitation before the engine picks up speed), that's called a "stumble", and it could be a bad accelerator pump (in the carburetor). But that's an under-rev on shifting, not an over-rev. With the car idling in neutral and the air cleaner removed, jazz the throttle to quickly increase the rpm while looking down the carb throat... you should see a very strong extra stream of fuel shoot into the throat while you are opening the throttle.
The strange thing is that you said the problem doesn't occur when the engine is cold. What color is the coating on the inside of your tailpipe after a brisk drive (black & oily, or powdery grey)?
