1992 Lite Ace Losing Power and Stalling

My partner's 1992 Lite Ace Diesel was having some trouble with low power and stalling, a local to her mechanic in Cambridge, MA did the obvious stuff and replaced the fuel and air filter and it runs much better initially but will still sputter and eventually stall after driving for about an hour, letting the vehicle sit for a while seems to solve the problem temporarily and saved us on a tow truck. Starting is also progressively more difficult if the engine is turned off for short periods. I'm told that a fuel pump issue would not allow the vehicle to run well initially so that has been eliminated as the culprit for the time being. Wondering if anyone has any ideas what would cause the engine to seem as though it were starved for fuel at the point that it should be warmed up and humming along? We finally found another mechanic willing to work on the vehicle as the initial guy we found was about to leave the country for an extended period when the problem re-surfaced an hour into a trip, hoping to come up with some ideas before we drop it off.