same thing happened to my 240D on the highway, worked great above 1/2 tank and hardly at all below - replaced primary and secondary filters and voila

__________________
1992 850i 6 speed
1991 M3 S54B32 swap
2007 328iT/6 RWD sport wagon
1987 325E 330iZHP engine swap
2004 Vespa 200GT
1976 W115 240D getting a much bigger engine. 5.5L
1997 R129 SL600 5AT