Check your fuel pump output -- I had an Audi Fox that exhibited similar symptoms (although it usually quit completely most of the time) when the fuel pump was going out. Worked fine until I got to Tulsa on a long trip -- croaked on the freeway in rush hour traffic in a construction zone. When it (and we) cooled off, it was fine.
Certainly sounds like a fuel problem.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!