You've described the classic symptoms of a stuck metering valve in the IP.
Step one would be to fill the fuel filter with a good cleaner. Start the engine and let it run until you hear a slight change in engine sound, maybe as much as two minutes. Shut the engine off and let it sit overnight. The cleaner will be in the IP doing it's thing. The next day start it up and drive it hard, drive it like you stole it.
Frequently this will free the metering valve and the problem will be over. However, this is often the first sign that the IP is getting old and will need replacement in the not too distant future.
What I've left out is the name of a good cleaner. We used to use ATF, but apparently the detergents have been removed from the current ATF. Some one will come along with the name of a good cleaner.