Your problem should be solved if you change or ultrasonically clean the fuel strainer. I have a similar problem on my 83 280SL and after cleaning it the problem was solved. When fuel level is less than 1/4 fuel flowing surface thru fuel strainer become smaller and the bottom of the strainer is clogged most. You should hear pump making noise from fuel starvation. Good luck.

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Roman
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83 280SL 145K (5 spd)
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