Had similar problem with fuel pump on AC 185. Fixed temporarily with electric fuel pump to boost pressure to input of injection pump. That worked for about a year then had to get fuel pump rebuilt. Works fine now.
Yeah, been thru 4 of them little lift pumps in the past 3 years. 2 were bad out of the box! These were AGCO supplied Holley. After the last one fried on energizing (dealer installed),we went with the electric pump used on older MF combines at 1_3 the cost. Still working after 4 weeks.
Could be restricted fuel return line. I'd get it to bother and then leave running while you remove top return line fitting on fuel pump flush with top cover. This will allow any contaminates (rubber pieces from drive in pump) to flush out. Then follow and blow threw line back to tank. It must have little to no restriction or you will get symthoms you describe. Good luck