A zone of highly irregular ice motion has been detected within the ice sheet adjacent to Jakobshavn Isbr¿, a fast moving ice stream in West Greenland. Two tilt sensors in different boreholes and frozen into the ice show angle variations of up to 0.1¿ and 0.3¿ within a 1200 s interval. The amplitude of these flutter oscillations increases toward the transition from Holocene to Wisconsin ice at 682 m depth, 150 m above bedrock. All other tilt sensors above and below show low background noise. A numerical model confirms that the stress configuration is exceptional in the neighborhood of the ice stream. Stress transfer from the ice stream to the margins causes an extra forcing of the basal ice at the drill site. Several possible explanations for the observed flutter oscillations are given, the most likely being dynamic rupture events or stick-slip motion along the Holocene-Wisconsin transition. If true, this would correspond to an active overthrust fault with very high loading rates.