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Check the fan. It might be dead, or prone to not actually spinning.
The fan would be found under a grille, possibly with rounded lines.
If it is overheating, something is making it overheat.
If it normally has no fan, then you may need to fan it.
Is the battery itself heating up? if so, that's a bad sign, and the battery may be shorted perhaps, or the laptop may be drawing excessive current from it, or the charging circuitry in the laptop may be defective and overcharging the battery (thus causing it to overheat.)
Another factor that can cause a laptop to heat up only *some* of the time, would be, if it sometimes has its CPU overloaded with tasks - e.g. having loaded several very 'rich' web pages with lots of ads and animations and pictures and videos playing on them, can overload a CPU like that, which makes the CPU chip hotter, which makes the whole region around the CPU (and its heat sink) hotter.