I have seen it before. Fuels leak back into the tank due to a minute leak around hose clamp. You can prove it by leaving the car standing over night and then remove the spin on filter. It should be full or near full. If you find it half full then it explains your program. The other thing you can try is to hang a fuel bottle HIGHER than the spin on filter and see whether it helps.

I found the minute leak near the fuel heater which is not easy to access or easy to tighten the clamps. I by-passed it. Problem solved.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.