The IEEE 802.3bj standard defines two potential low power operating modes for high speed energy efficient ethernet (EEE) physical interfaces working at 40 and 100 Gb/s: a not-so-efficient low power mode that requires very short transition times to restore normal operation (Fast-Wake) and a highly efficient low power mode with longer transition times (Deep-Sleep). In this paper, we present a new frame coalescing mechanism that dynamically adjusts the coalescing queue threshold in order to minimize the energy consumption of dual-mode EEE interfaces and maintains, at the same time, the average frame delay close to a target value. The proposed mechanism has been validated through simulation under different types of traffic (Poisson, self-similar, and real Internet traffic). In addition, we show that, with the current transition times and efficiency profiles of the standardized low power modes, our proposal renders the Fast-Wake mode unnecessary in most practical scenarios.