Isotopes are considered stable when the strong overcomes the electrostatic repulsion between force positively charged what?
When the strong nuclear force between nuclear protons and neutrons overcomes electrostatic repulsion between protons.
You might get a more nuanced answer in the Physics section, because the strong nuclear force is something that chemists don't have much of a handle on. As far as I know (and certainly my understanding is not sophisticated!), in the nucleus there is an exchange between the massive particles, protons and neutrons, such that the electrostatic attraction between like charges is overcome, and an attractive, binding force operates. This binding interaction is VERY short range; i.e. we talk of nuclear distances.