Overrun = off the throttle where the vehicle's momentum is driving the engine and usually decelerating, unless on a downward slope where, depending on selected gear, it might be holding speed or even accelerating. But the engine is holding the vehicle back one way or another.
It implies that the loads and torques are reversed throughout the engine, transmission and driveline compared to being under power.
It also implies the engine, although maybe spinning fast, is running on a low or shut throttle and fuelling through the idle jets on a carburrettored engine or with its fuel completely cut-off on most injected engines.
As it stops mucking about imemdiately you lift off and re-apply power, its unlikely to be icing up, and would be more apparent in certain humid and dewy conditions. To diagnose it you would have to stop quickly, and look down the carb for signs of frost. Maybe worth checking that the carb heater is working though, if it has one.
