First thing to do is to replace the fuel filters, both pre and main. Check that there is reasonable flow from the feed pipe - i.e., that the tank strainer isn't blocked.

It's allways worth replacing the O ring seals on all the low pressure fuel lines, and rnewing the injector leak off pipes, to avoid fuel leaks, and to eliminate air from the system.

If the problem persists, then, get the injectors out, and do a compression test. While the injectors are out, get them tested for opening pressure, dribbling, and spray pattern.

Unless abused by a previous veg burning owner, the IP is extremely unlikely to be the root cause of the problem.

The valves on OM606s are self adjusting, driven by hydraulic tappets, so, there's nothing to adjust. However, valve seat recession is not unknown, but, this will be picked up by the compression test.

[If I were a gambling man, I would back that the answer will come from the compression test]