Yup, NMOS chips did suffer from thermal runaway, but they're very very rare nowadays, and in any case, modern chips are generally throttled back by external clocks so they couldn't suffer this fate. Some NMOS chips used to use internal clock generators or frequency multipliers/dividers, and as they heated up, propagation times dropped, making them run faster which generated heat, and thus creating a spiral of doom for the chip ending in burning out the core. CMOS chips are the opposite, propagation times are longer with higher temperatures so this can't occur, but the fact that they're nearly always externally clocked anyway makes it a moot point.
