You might also want to check for vac leaks in the engine compartment -- I had to replace all the rubber connectors on my 300TE to cure the tranny shifting problem. Worst was the idle control valve hose -- rock hard and loose. Don't know if you have that, but if the vac supply line is loose, it can do strange things with the servos.
Don't know why that would cause your trouble, though -- should have higher vac at idle than at 1500 rpm.
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!