My phone keep uninstalling my apps and sometimes deletes data on sdcard

I use a wiko slide 2, the phone comes with a 16GB ROM and a 2GB RAM and Android 5.1 lollipop.
Though from the onset, I found the RAM performance to be below expectations, I didn't observe it was deleting my apps.
Few months after I started using the phone, I discovered that some installed apps are being deleted without a trace. The phone randomly selects apps to uninstall I guess.
This has given me concern.
Just recently, the phone deleted a large portion of my downloaded media (from the download folder only), and wouldn't accept to save any type of files from any source into the SD card.
This condition lasted until i unmounted the SD card, removed it from the phone and reinserted it.
Today, the same even reoccurred.
I have scanned the phone severally but all the antivirus I used would report that the system is healthy.
What so I do?