this same thing started to happen to me about a year ago and after looking in both the engine compartment and the fuel tank I gave up for a while and just kept my tank as full as I could. but it steadily started to get worse until i could barely drive anywhere, regardless of the level in my tank. I ended up finding out the fuel lines from the tank to the engine were rusting out. with a full tank there was enough pressure in the lines from gravity to not allow any air to suck in, but they sucked air at lower levels of fuel.
I did not have to replace the entire length of the fuel lines, I just cut out the rusted part and replaced it with some flexible fuel hose (aprox three feet total). this did the trick. the most common place for this rust is in the rear end above the sub frame where salt and debris hangs out, you can check this by looking around the driver's rear tire for fuel drips- these drips started very slowly on mine so they were hard to find. it helps to clean the fuel line off with a rag so it will encourage the fuel to glide down the line and drip on the ground just infront of the rear tire. if this ends up being the problem check your sub frame for rust too-very dangerous situation if they start to rust through.
as I have been writing this I have also been thinking that you should check your overflow tank in the spare tire compartment and make sure that is not leaking any fuel there or any of the hoses from the overflow tank to the main tank. I could see this sucking air at certain levels of fuel while not sucking air at others.
to test if your system is sucking air, the next time you lose power pop the hood and inspect your fuel filter if it is not very full that could be an indication it is sucking air. with the engine running vigourously pump your primer pump and quickly drive a mile or two to see if it fixes the problem temporarily, if it is really idling hard pumping the primer pump should immediately smooth out the idle as well.
After I diagnosed my problem and drove to the shop to buy parts I ended up having to pump the primer pump every couple of miles just to get down the road (they are back roads, so it was o.k. to drive 10 miles an hour with the pedal floored!)
when I was checking my fuel filter it had little black flecks in it, which I found out were tiny rust flakes coming off of the inside of the fuel line. this was fixed when I replaced the bad section of fuel line so that might also be an indicator
anyway hope this helps, if it is not caused by sucking in air I don't know what the problem could be.
cheers
Scott

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Thanks! Scott
sdpettitt@yahoo.com