L175 shut down help

Morning folks, I have gone through the forum and got some great advice
but have run out of ideas. Need some insight if it's available. My old man picked up a L175 at Auction. Engine needed rebuilt and it was. Starts up just fine. The issue is that it shuts down no matter the rpm after about 5 mins. First assumption is fuel starvation issue, done bleeding, cleaning etc. There is no fuel starvation issue. You can pull decompression open and turn engine over with injectors cracked and you get good flow from both. Pulled line off at injection pump also and tested flow into a can. Can't find a fuel starvation issue. Last night I removed injector pump. Seems perfectly fine. The system is so basic I can only figure it has to be an issue with injector pump. Would you come to same conclusion? Basically there isn't anything else to look at. Unless gov issue but that's intact and seems fine.

Any advice? By the way after it shuts Down you can start it right back up and it's like you reset a timer for 5 mins

I appreciate that guys. Hadn't thought of it like you guys did. I did suspect flow issue and that's why I pulled hose at pump and checked. Drained about 1/2 gallon with no issue. However don't mean anything most likely cause I was allowing flow at higher rate than system would pull. I got one other tid bit from another thread with kinda same issue about making sure return line isn't clogged. Honestly didn't check return for flow.