My apologies for the delay in getting back to you, Ive just returned from dinner and running a few errands.
I agree with your concern 100%, a returning issues usually does signify an underlying issue. The steps you mentioned all seem to be solid steps. Can I ask if you h ave tried a permissions repair and disk repair using disk utility?
Lets try both, heres how to get there.
Open your finder ( magnifying glass ) type in disk utility, and launch the disk utility application.
On the left side of the disk utility window, click your mac hd, then at the lower right hand side, click repair permissions .
Once repair permissions is complete, we next want to try and repair disk, and let that complete.
Avast is free, and should be able to run on Leopard. https://www.avast.com/free-mac-security
Let me know if you encounter any issues installing it and Id let it run a scan overnight just to be safe.