[Lloydster's 85 6.9 IDI]

Only 1st start of the day, I have to dump a cup of diesel in the intake to get it started. I was using starting fluid, but was told its harmful. So I tried straight diesel and it worked! It starts fine after that, unless it sits for more than a couple hours.
So, which component is losing pressure? Diesel pump 😭 (sound of wallet screaming), injectors😢 (more screaming), or fuel return supply/return lines?
What should I do first?
Thanks for your suggestions!
'85 f250 6.9 IDI 2wd longbed xtra cab "Stretch"