1997 Ford Thunderbird rough idle.

This car has been well maintained. Even with high miles when I bought it a few months ago it was running great. I did notice a slightly low idle. The idle has become erratic. There is some system that seems to be something kicking in and bogging the engine down at low idle. And the car will not idle steadily. It almost dies and then it goes up to about 1200 rpms and then bogs again. In dr if I were to sit with my foot on the brake and slightly press the gas to keep the RPM's up just a little it doesn't do it. Its only with my foot off the gas in idle. I cleaned the mass airflow sensor properly, but I have absolutely no acceleration issues. Just idle. So I replaced the idle air control valve. Its still doing it except the car won't die it just runs erratically. I'm missing something here and I don't have the money to go out and spend on a mechanic. I barely have the money for the parts. It pulled a code for one of the oxygen sensors. I replaced that. And it pulled a code for a different one. I have yet to replace that. I would like to know what other sensors or systems I should be looking at to try to resolve this issue. Like I said the car runs great when you're going down the road. It just won't idle properly. I look forward to your advice and response. Thank you very much for your help. Don