We have had the Uniden PowerMax 5.8 cordless telephone with 5 units for about 10 years and have been very happy with it. We have replaced the rechargeable batteries. Recently, the 2 units farthest away from the base unit are exhibiting spotty service with lots of static on the line and occasionally displaying "out of range' on the readout. A lot of times, it is worse than a bad cell phone connection. What may be causing this and how do we fix it?