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Just to clarify here for me, you had a rough idle at 1/4 tank but when you put in more gas the problem would go away?
Now you are having that problem no matter how much gas is in there?
This indicates that the fuel pump is going bad. The fact that the car runs fine when there is 1/2 tank, but not 1/4 tank indicates that the fuel pump is weak. The fuel pump sucks in fuel, if it's weak it need additional pressure to be able to suck the fuel and delivery it to the rails. With more gas in the tank there is more pressure exerted on the pump, allowing it to pump the gas with less efforts. You may consider doing a fuel pressure test on this car when the fuel is low, this is verify if the fuel pump is infact bad. You also may consider checking and replacing the fuel filter.
Yes even it the car is idling normal the fuel pump still may be going bad. The car requires the least amount of gas during idle operation. The fact that the problem was there while the tank was at 1/4 and not there at 1/2 indicates that the fuel pressure is having an issue. This is most likely due to a bad pump and is having trouble pumping gas, more gas in the tanks makes it easier for the pump to work.
Well you can run a quick test on the intake gasket. Here is how you would perform that test.