Sounds like a fuel delivery issue. Could be air restriction too, on inlet or exhaust side as 745 said, but I think you would notice a lot of black and gray smoke if that were the case. Enough of a restriction to hold the engine at 2500 rpm when it's getting full fuel delivery should mean a big smoke show.
Do you have your fuel can running direct to the IP inlet, or through the old fuel filter? If the old filter is in the system that could be the issue. Generally good to try avoiding running with no filter in the line since even a little bit of dirt can hurt IP and injectors.
"In" and "out" banjo fittings in the right places on the injection pump? Return line not blocked? The banjo bolt for the return fitting at the IP also has a little screen inside it, if the pump had something get dislodged and then get stuck in that screen and plug it, can cause strange issues. Might take that off and see if it is clear.
Could have some injectors doing weird things too if they were sitting forever with old fuel in them. E.g. leaking compression in above certain engine speeds, etc.
Could even be a timing issue -- like if the advance piston in the IP is stuck in place and is not giving any dynamic advance as engine speed increases, it might get to a point where it doesn't have enough timing to run faster. If it sat for a really long time with the same fuel in it, I'd wouldn't make any bets on the pump and injectors being 100% happy.
You might try running it and letting it fill the pump and injectors full of biodiesel or Diesel Purge or ATF or whatever you like as a detergent/solvent and let it sit for a few days, then see if something frees up.
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