I have looked this up and although there are several threads that have similar issues the issue i have seems to be a little different in some respects.

I started out with my 2000 F150 with a 5.4 V8 running rough at idle. It would idle way down slow and then rev up a little over and over. On a couple of occasions it would even die when in drive or reverse. I did finally get a Check Engine light so i put it on my diagnostic program to see what was up. Here is what I got:
P1151 Lack of HO2S21 Indicates Lean
P0171 System Too Lean Bank 1
p0174 System Too Lean Bank 2

After doing some reading up I checked all my vacuum hoses and cleaned my mass air flow sensor. I then got the same codes again after a few minutes of running. So I went to the next thing that was recommended, the upstream bank 2 O2 sensor. After changing it the truck ran fine for a couple of days, no issues. Then all of the sudden it idled a little funny again and I got a Check Engine Light again. This time:
P0171 Bank 1 Lean
P0174 Bank 2 Lean
P0300 Random/Multiple Misfire

I reset the light to see if I would get the same again later. Before the light came on again I filled my truck up with gas and it seemed to be running pretty good again. Then a day or so later it set the light again. This time:
P0171 Bank 1 Lean
P0174 Bank 2 Lean
P0136 O2 Circuit Malfunction Bank 1 Sensor 2
P0156 O2 Circuit Malfunction Bank 2 Sensor 2
P1131 Lack of Switches HO2S11 Lean
P1151 Lack of Switches HO2S21 Lean

I went ahead and drove for a few days as it had gotten really cold here and I didn't feel like messing around in my shop since it isn't heated. A couple days ago I needed gas so i filled up again. Suddenly the problem was gone again. Truck ran fine idled fine no missing or odd idling at all. That lasted a day or two before it started acting a little weird again. At that point i was down about 1/8 of a tank from full. I have read that there can be issues with the fuel pump.. I am kind of wondering if that may be it now. But I haven't had to deal with this sort of issue before so I am unsure what to check next. The truck always runs fine off idle, it only seems to have the issue when it is idling. I had reset the codes the last time and i have the light on again now. I haven't checked it yet but I am sure it is going be some combination of what i have had in the past.

Any Help is greatly appreciated!

 

First I would change the fuel filter if it hasn't been done in a while. Next check the fuel pressure, it should be in the 30's. I see you have already checked for vacuum leaks, but make sure you check the PCV elbow on the back of the intake, if you haven't already.

 

While your checking all the above get a can of starter fluid and with the truck at idle spray around the intake manifold and listen for a rev of the engine.

 

I've checked the elbow. It is good. I just put a gauge on it too to check the vacuum and its at about 22-23 inches. I don't have a fuel gauge to check that with although i am working on locating one at the moment. Of course i am out here in the cold working on it and it is running good now...LOL.

I did forget to mention that a long time ago i had an issue with the idle air bypass valve. It stuck a couple times and i cleaned it and didn't have any issues with it afterward. I don't think that is what it is this time, and i have cleaned it recently to make sure it was ok. Plus my Scantool is saying most of the codes it is getting are at highway speed and not when it idles, although the problem is only there when it idles. Also I never got any code when the bypass was stuck either.

Im gonna get a filter as it has been a while. Is it possible that since it is getting the codes at higher speeds that it is then trying to correct for the issue and it really is only noticeable at lower RPM?