Either fuel flow TO the injection pump is retricted, OR the fuel return TO the tank FROM the injection pump is restricted, most likely by a piece of the governor damper ring that has disintegrated into "coffee grounds", and plugged the housing pressure control valve/fitting that is screwed into the injection pump's cover.
You can remove the fitting, and clean out the crud (caution, there's a GLASS check ball in there), and it will run 'til it clogs again, if that's the problem.The fitting/check valve in question is part Key #4 in this illustration. (Courtesy of Deere, but similar to your's.) 
CAUTION... continuing to run the tractor for many hours without having the IP repaired will make governor parts run "metal-to-metal", once the ring completely disintegrates, allowing the metal parts to rattle together, and put metal filings in the fuel system. Key # 16, in the drawing below, is the plastic ring that fails over time and disintegrates. 
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