Multiple, changing ECU codes

So I recently got the listed above for a good deal. Okay, a really good deal, as in free.

It was running rough, with a check engine light. I replaced the igniters and plugs and problem solved.

A week later the check engine light came back on. At auto-zone they told me the light indicated replacement of the mass airflow sensor. I did so (very easy procedure BTW) and all was well. For a few days.

This time the code said that the bank 1 O2 sensor was bad. I replaced that (not as easy, but fairly straightforward) and the light went away. For a couple of days (about 500 miles, I went on a short trip).

Now another check engines light, again telling me to replace the bank 1 O2 sensor.

I'm starting to get the feeling that this might be the computer itself and not all these components I keep replacing. Currently I'm just going to pull the O2 sensor and return in (assuming I got a dud), but. Who knows.

My thoughts were to re flash the whole computer. The only information I can find is that yes, you can do it yourself, but only after emptying your bank account to buy the software from GM. Is this true or are there more, ahem, covert ways to go about this without having to pay GM because the car they built broke?

BTW, the engine runs absolutely flawlessly. I can't imagine it running better at it's age and mileage, it's just I'd really rather have the check engine light off so it can alert me to a real problem if one comes up.