Runs better on low fuel?

[GRIESL]

I promise this isn't like the "the car runs better after it's washed" theories. My car honestly runs better with five gallons of fuel or less. Any thoughts why that might be? I've speculated that it may be because the fuel is being circulate more through the return lines which could heat it up, or possibly filter the fuel more. Or is it symptomatic of a problem that exists? i.e. lift or injection pump? Thanks in advance.