If you suspect the pump and it may be a possibility in my mind. It is possible something may be sticking a little. Like the rack.

You might entertain the possibility of good injection pump solvent soaking in place. There are two sections internally. So you have to soak the section where you suspect the problem might be. Or both.

Some pumps that are just seriously gunked up internally have been found. Especially those that used alternative fuels like waste vegetable oils at one time. Also keep in mind that a twenty percent difference in power is also just about consistent with a cylinder on and off line sporatically.

One quick test is to close off the return line from the injection pump and test drive. A gummed up injection pump relief valve might open and be slow to close. Reducing the fuel supply pressure in the injection pump until it does.

These two things mentioned are not high probabilities. Just remote possibilities. As your symptom in general is. I do not think over the years we have even had another posted complaint identical to this one.