Does it happen just as you tough the brake, or does it happen at a fixed rpm as you decelerate? If when you touch the brake, you probably have a leaking brake booster.
If not, it's something with the fuel injection -- verify that the EHA current is correct and not ocsillating -- if it goes way negative (fuel cutoff) then bounces rather than going back to near zero, check the duty cycle and verify correct mixture, and check O2 sensor function. You could have a slow O2 sensor causing mixture flucuation on deceleration.
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!