There has been many discussions on this issue with many ideas of causes and solutions.
Here's what worked for me. Download an app called
StorageTools and format the SD card to FAT32. I found this stabilized my card.
I also found that when I delete items from the card it doesn't seem to release the space properly, causing 'out-of-memory' errors. Using the same app to defrag the card cures that problem. To defrag the card you'll need to purchase the app as defrag doesn't work in demo mode.
Chris...