I reread your first post and have second thoughts - but no remorse.
When driving at a 60 MPH constant speed the engine suddenly lags but the
lag can be overcome by pressing the accelerator.
That means there's lots of fuel to the injectors or the lag couldn't be
overcome by accelerating the engine. It's not a fuel problem.
I suspect the emission control system which, on a diesel, is mainly the
EGR valve. As a test I'd plug its line and see if the problem showed
itself. If not, the controls for the EGR would be my first suspect for
after 87K miles a bit of dirt could have lodged in a vacuum line or
control valve or a rubber connection may have split causing erratic EGR
This car has a 5 speed electronic transmission with a lock-up torque
converter. The car might FEEL like the engine was lagging if the lock-up
disengaged, but the engine is not the culprit. If the lock-up feature
failed, the engine's RPMs would rise about 3% to maintain the car's
speed. The lock-up feature gives one a "direct drive" feel, especially
in 5th gear which is an "overdrive" ratio. I own a '97 E320 and can
imagine how this would feel but have no personal experience with a
lock-up failure. The test of this theory is if the engine's RPMs rise or
fall when you perceive it lagging. A RPM rise indicating lock-up
failure, falling RPMs true engine lag, perhaps due to EGR.
So those are a couple of ideas for your consideration, I hope one or the
other solves the problem.