This is not normal. Once a transaction is confirmed it should stay in the list and the balance should not change unless you use some coins.

My first throughts are either the wallet file got lost due to disk corruption or your computer is infected.

Is the computer stable or does it crash ocasionally?
Do you proactise good computer hygiene of not running software from untrustworth sources and have a good up to data antivirus program.

Perhaps people will have more ideas if you say a bit more about your setup start with what operating system?