CEL RF modules

Has anyone had experience with CEL's line of RF modules? We are trying to spec in CEL ZICM2410P2-2 and are having trouble getting the modules to go to sleep, and/or stay asleep, and/or sleep at or close to the advertised current rating of 15uA. Some times they work, sometimes not. We are driving the modules low through a Micrel voltage supervisor MIC2775-26YM5. Test equipment shows the sleep timer start running (current consumption increases slightly) while the module is in a "ready" condition. Then current begins falling like it is going to sleep. As the 15uA level approaches, current begins to waver, then steadily climbs to around 500uA. I have replaced the supervisor, both across same part number, and different models, with little to no change. Any ideas?