Any way to reduce the frequency of searching for a signal
I had similar problems and used Llama, then expanded my use of it to cover other events. It is similar to Tasker, and sufficient to my needs, while still a free app. Most automation can be triggered by entering/leaving cell tower range that is recognized as "Home". It has options to turn on Airplane mode automatically when the phone leaves a cell reception range, and take it out of Airplane mode when it re-enters.

BTW - I'm just an enthusiastic user - not the developer.