It runs but shuts off when it's driven,unless I keep my foot on the accelerator

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Hello, I have a 1993 Isuzu pick up standard 4-Cyl, 2.6 Liter , 5 speed manual transmission. We just replaced the catalytic converter, it passed emissions,as soon as it passes, it started shutting itself off , the day it passed . It runs and stays on if you just let it sit there and idle,but if you drive it for a few minutes, it shuts off. It starts right up, but will continuously shut off unless you keep your foot on the accelerator.