Over the years, a considerable research effort has been applied to the design of ad hoc network routing protocols. However, there is still a lack of understanding of the subtle interactions between routing protocols and lower layers in the protocol stack. In this paper, the instability which may arise when reactive routing protocols interact with the IEEE 802.11 MAC protocol is investigated. In particular, several erratic behaviours of the Ad hod On-demand Distance Vector (AODV) routing protocol in a congested IEEE 802.11 ad hoc network are demonstrated. A cross-layer solution is proposed based on an Adaptive Bulk Trigger policy and a Dynamic Window Selection scheme. Simulation studies are presented which show that the proposed solution is effective in alleviating erratic behaviour of AODV and improving the end-to-end path stability.