A model of a particle interacting with two periodic potentials, one of which is externally driven, is analyzed. Three regimes are identified in the motion of the driven plate: (a) stick-slip motion, (b) intermittent stick slip characterized by force fluctuations, and (c) sliding which occurs above a critical driving velocity vc. In the vicinity of vc the power spectra of the force obey a ω−2 law and the force fluctuations decrease as \(vc-v\)1/2 for v < vc. Our calculations suggest that stick-slip dynamics is characterized by chaotic behavior of the top plate and the embedded molecular system. An equation is derived which provides a coarse-grained description of the plate motion near vc.