If the problem begins again after a clean system and software install, the next possibilities are:
2. Your hardware is corrupted. 2A. Worst case. Your disk might be bad. Use the Disk Utility functions to test it.
Bad disks are far rarer than they used to be, but not unknown.
If your disk tests bad: You have backups. You will need to get a new disk. You can use an external disk (my preferred backup mode) to toddle along until you can afford to/take time to get this. If you are still under warranty/Applecare, Apple owes you a new disk. (They may fuck your computer up putting it in, but I do not recommend that you try this yourself unless you are experienced in playing Operation and in taking apart interesting fiddly bits of computers.) Absolutely have backups, backups, and more backups before sending it off to Apple. If your disk tests bad and you do not have warranty/Applecare coverage, buy a new computer.
2B. Very worst case. Something on your board is bad. I had a lemon 700 that had a bad processor---and apparently 90% of 700s had a bad processor. It subtly, slowly corrupted everything running through it, in an iterative process, until the file broke. I had weird bugs like dictionary entries disappearing, files vanishing, crashes without end. It took more than a year for Apple to admit this was unacceptable. They did send a new computer after breaking, successively, the latch, the Airport antenna, the screen... all whilst trying to fix CPU that was bad by replacing the disk, the motherboard, etc. Went through three motherboards. All bad.
A bad chip is nearly impossible to detect with standard testing utilities, and no one does component-level repairs.
So, if the other stuff does not work, buy a new computer. Seriously. Use it while you badger Apple to fix the old one. Apple will not provide a loaner. Buy a refurb from them; they're cheaper than new and better-tested, so more likely to be solid. Sell it if/when you get your computer back in working order. I'm quite serious. I assume this is a tool you need every day.
If you still have Applecare/warranty coverage, you can call Apple and spend some time with their tech support people. Keep copious notes of anything, however trivial, they say to you; note down any tests they have you do and the results. This can save time if you have to call again and they try to run you through the same hoops.
exceeded comment length, sorry, here's the rest
If the problem begins again after a clean system and software install, the next possibilities are:
2. Your hardware is corrupted.
2A. Worst case. Your disk might be bad. Use the Disk Utility functions to test it.
Bad disks are far rarer than they used to be, but not unknown.
If your disk tests bad: You have backups. You will need to get a new disk. You can use an external disk (my preferred backup mode) to toddle along until you can afford to/take time to get this. If you are still under warranty/Applecare, Apple owes you a new disk. (They may fuck your computer up putting it in, but I do not recommend that you try this yourself unless you are experienced in playing Operation and in taking apart interesting fiddly bits of computers.) Absolutely have backups, backups, and more backups before sending it off to Apple. If your disk tests bad and you do not have warranty/Applecare coverage, buy a new computer.
2B. Very worst case. Something on your board is bad. I had a lemon 700 that had a bad processor---and apparently 90% of 700s had a bad processor. It subtly, slowly corrupted everything running through it, in an iterative process, until the file broke. I had weird bugs like dictionary entries disappearing, files vanishing, crashes without end. It took more than a year for Apple to admit this was unacceptable. They did send a new computer after breaking, successively, the latch, the Airport antenna, the screen... all whilst trying to fix CPU that was bad by replacing the disk, the motherboard, etc. Went through three motherboards. All bad.
A bad chip is nearly impossible to detect with standard testing utilities, and no one does component-level repairs.
So, if the other stuff does not work, buy a new computer. Seriously. Use it while you badger Apple to fix the old one. Apple will not provide a loaner. Buy a refurb from them; they're cheaper than new and better-tested, so more likely to be solid. Sell it if/when you get your computer back in working order. I'm quite serious. I assume this is a tool you need every day.
If you still have Applecare/warranty coverage, you can call Apple and spend some time with their tech support people. Keep copious notes of anything, however trivial, they say to you; note down any tests they have you do and the results. This can save time if you have to call again and they try to run you through the same hoops.