We report the results of experiments on a "fastbeam-ion instability"[1] at the Advanced Light Source (ALS). This ion instability, which can arise even when the ions are not trapped over multiple beam passages, will likely be important for many future accelerators. In our experiments, we filled the ALS storage ring with heliumgas, raising the pressure approximately two orders of magnitude above the nominal pressure. With gaps in the bunch trainl arge enough to avoid conventional (multiturn) ion trapping, we observed a factor of 2-3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. Show Partial Abstract