Have you cleaned re-gapped the points? If not give that a shot. Gap to .020" If they are oily when you pull the cover off you will need to replace or add a plunger seal. Some older models didn't come with a seal.
If that doesn't work get a new set of points, new condenser and a new plug. If after you do those things you continue to have an issue of it running till it warms up, then losing spark it's time to buy a coil.