As everyone has already mentioned; it's always good to start with the basics like fuel and air filters, perhaps a Diesel Purge, and the banjo bolt and adjoining line. A valve adjustment may be in good order as well, but it sounds as if this power loss was somewhat abrupt, still, properly adjusted valves are always a good and important part of a 617's maintenance regime. I have run into the clogged banjo bolt problem on my, as well as my friend's, 617 turbo rigs, and every time I have had a tangible gain in performance and efficiency simply by cleaning the bolt and line...just be careful not to crack the old, and likely birttle, line that runs from the bolt.

These things may not solve the problem, but it's a good, and cheap, place to start.