Just read this whole thread, glad I got here before it got any bigger .. This issue sound exactly like mine. My idle was really bad last weekend in traffic at the gold coast - temp probably over 30 deg, even at 800 rpm idle I got irratic idle. If I set my idle to 900rpm I don't notice any problems at all. If I set it to 700 rpm I would be lucky to get around the block without a problem. Cold days usually fine. With summer just around the corner my idle going to have to be around 850 rpm to avoid the problem.
One thing I do notice is when I let go of the accelerator somethings it will stop at 1000 rpm and gently go to idle and sometimes it just falls past base idle rpm and then finds idle. If I knew what controlled the stopping at 1000 rpm then that be a problem.
I just replaced my O2 sensor with a ford el one, still didn't fix my problem.
To try and find the problem, I reckon one way to tackle it would be to use a datascan software and record when your idle is running well and when your idle is acting up and compare the two. You should see a difference somewhere in the log. If I had a laptop I would do this myself.