1974 INT. 4300, cummins NTC 350 repeated engine shut down

Engine shutdown for no apparent reason at very low speed low rpm. Like shifting from 1 to 2 or 2 to 3. Idles perfectly. Restarts perfectly. Will run for several minutes at low speed before doing this again but always when under load. I haven't run it on the road at high speed since this started for fear of getting stranded on the road but had been running good at all speeds up until today. Thanks

1974 INT. 4300, cummins NTC 350 repeated engine shut down