No, it's probably not the ballast resistor. With a faulty ballast resistor, the car won't start (most often it'll start but immediately die when the key is released from "start" to "run"). That "heat up and increase resistance" thing happens about once every two or three nevers. If the engine runs at all after the key's released, the problem's not the ballast resistor. The coil is probably not the problem, either -- they can fail, but they seldom do.
Heat-related quality-of-running problems are almost always carburetor/fuel related. If this were a heat-related ignition/electrical problem, the engine wouldn't run (or would run very poorly at all speeds) once it got hot. If the condenser fails, the engine runs poorly (at all speeds) or not at all.
So, while the ignition-related tune-up replaceables (points, condenser, spark plugs and wires, cap and rotor) ought to be kept in good condition -- Tune-up parts and technique suggestions in
this thread -- focus on the carburetor first to fix this specific won't-idle problem. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted
here for free download. Problem or no problem with it, the carburetor's idle mixture and speed must be adjusted correctly (info at same link).
The PCV valve also affects the idle more than higher engine speeds; if it (or its hose) is faulty or clogged it can cause a no-idle condition. Moving further down the list of things to check: if the
valve adjustment is too tight, it can cause a cold-idle-hot-run-but-not-high-idle problem.
