Digistat RF3 losing connection?

I have been running Digistat RF3 wireless programmers on my combi system for about 10 years. About every 5 years it seems that the radio signal weakens and no longer reaches the receiver. When this happened 5 years ago I replaced both units which fixed the problem, now it has happened again. If I move the sender closer to the receiver it works fine. I have tried setting it all up again from scratch with no difference. Batteries are new and there is no corrosion on the contacts. Is it true to say that these units have a life of about 5 years or I have I just been unlucky?