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Usually takes a little farther than a half mile but if your gas cap is plugged you might be creating a vacuum in the gas tank and starving for fuel.
Drive with the gas cap off to see if that solves it.
Rubbing block on your points may have worn down and now the point gap is too small.
Check gap and re-set if necessary.
A few swipes with some fine sandpaper across the point surfaces before setting them would be good.
Spray some starting fluid on all of the manifold gaskets while engine is idling to see if you have a change in engine speed. If you do you have a leaking gasket.
When working through this do one thing at a time and test drive. That way you will know what the fix was.