Before you go to the trouble of checking the tank screen. That is a good suggestion though. Just take a minute and make sure the cigar hose is present in the engine compartment and has not been substituted with a hard piece of hose.

I think your symptoms are reversed but a member had a hard hose substituted for the cigar hose and at the transition point where the return line in the fuel tank was uncovered the car ran differently. It was a hard sell to get him to install a cigar hose but it really cured the problem.

If this problem has developed after your ownership than it cannot be the cigar hose . As also sugested I cannot remember what his exact situation was so much time has elapsed since he had the issue.

A quick check of the condition of the tank screen filter. Remove the fuel line at the back when your tank is low. Is fuel still flowing freely by gravity from the tank? Also after you check your low tank fuel delivery performance.

If it is okay I might try closing off the return line from the injection pump. With no resistive fuel load when the tank is below the return line there may be very low fuel pressure in the base of the injection pump. When the fuel level is higher the additional restriction might just increase available fuel pressure a little. Long shot by the way but easy to check. Would almost be symptomatic of a constantly open relief valve on the injection pump by the way.