It's a little (!) difficult to diagnose issues at a distance, but this sounds as though it could be a memory/storage issue. I don't know that particular model and it sounds as though it might be quite old, in which case it'll be out of warranty so a fix might be more expensive than a replacement. It's also possible that the available storage has filled up over time, so I'd start by deleting as much as possible: any content (eg music, photos, if applicable) that you don't need and also old text messages. Some devices automatically clean those on a first-in-first-out basis and some don't.
There shouldn't be a system policy to remove contacts in the same way, but devices sometimes behave oddly when they start running out of space.
