Power loss under a load

[Ben300SD]

I have been intermitently experiencing this problem since April of last year, but in the past it has rarely manifested itself. Two days ago I replaced a fuel injector and washed and degreased my engine and the problem is back. Here are the symptoms:

When driving my 300D sometimes the I feel a strong loss of power. This usually happens at speeds greater than 60 mph, or at lower speeds if the engine is revved up high (>3000 rpms). Sometimes the engine does not regain power for a while, other times it surges, other times it is a momentary thing and everything returns to normal within a second. The best way to describe the feeling is to say that it feels like the fuel supply is being pinched. It feels like a fuel problem to me, but I am not a trained mechanic.

How should I go about dianosing this problem, and what should I do to fix it? Any help will be greatly appreciated, as I am taking my car on a road trip in the next month or so. Thank you.