Hey Wally!
Sounds like a vac prob to me. There is a lot of tuning that you can do on the vac system on one of these. The modulator is the best! Adj, drop off jack, test drive. Jack, adj, drop, test drive... etc...
There is plenty of info on here, but I have some links if you have any trouble finding it. You will know
way more than you ever wanted to know about diesel vac systems by the time yr done.
Cheers, and happy dieseling!
-Matt-