Many thanks for your response. I've looked through these articles and tried the things which look like they might apply (short of reinstalling the OS, which scares me), but the problem persists.
The bit that I think is unusual, and which I've not been able to find information about despite googling for several hours, is this business about being unable to turn wifi back on again once I turn it off, without restarting the machine.
The wifi failings are becoming more frequent; I just suffered several within a fifteen minute period.
Grateful for any furthe help.
Did you try setting up a new admin account and testing there?
If you live near an Apple Store, make a Genius Bar appointment to have the computer tested. Supposedly there is no charge for testing.
Thanks for your continued help. I had tried running in Safe mode (as my normal user) and found the problem there too, but a new Admin account currently seems stable. I'll continue to use it over the next few days and see how it gets on.
Could you tell me how I would go about transferring all my data between user accounts, if this turns out to be the solution? Is it simply a question of moving everything into the 'Shared' folder, and then populating the new account from there? Am I likely to encounter permissions issues? Obviously I don't want to do anything that will replicate the problem in the new account.
Is it simply a question of moving everything into the 'Shared' folder, and then populating the new account from there? Am I likely to encounter permissions issues?
That is how it is normally done or you can put the items in the Public folder of the new account. You might encounter some permission errors.
Based on what most posters recommend, it is normally better to resolve the problem and then upgrade. Some problems get carried forward.
If you like to see if it would be better using Mavericks, one option is to create a new partition (~30- 50 GB), install Mavericks, and ‘test drive’ it.
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Updating for the sake of any who have this problem in the future and find my thread:
I took the computer to the Genius bar. They ran diagnostics and determined that my Airport card was OK - but then when they rebooted, it wasn't found. After reseating the card, the computer worked for a couple of days, but it failed again.
The Apple Store then replaced the cable which serves the Airport card, which was apparently showing signs of wear. They weren't sure that it would solve the problem, and didn't charge labour since they thought I might well be back for a new Airport card shortly. But I've had no problems in the week-and-a-half since that repair was made, so I'm considering the matter solved.