Issue present immediately when trying to restart after shutting down or only if it sits for a few minutes? How many miles on the motor and what's the history?
Hard restart after a short sit when warm is pretty common to see on one of these as they age. Bet all the times it is easy to start are the times when you can hear the GP relay click on (even if only briefly), and the times it doesn't want to are when there is no GP action? And furthermore, I'll wager that if you reach under the hood and pull the GP thermo signal wire off the sender in the back of the head, then glow for a second or three, it fires right up?
IDI motors need glow plugs more of the time than DI setups even under the best of conditions (i.e. compression to new specs, cranking speed correct, timing and valve clearance all OK), and as the motor ages or is affected by other factors over time (e.g. weak starter, battery, batt cables/terminals, ground connections etc) and begins to need glow to start more of the time, the programming logic of the glow controller slowly becomes inappropriate. IOW, when cold it glows and starts easily, warm just after running it's hot enough to start easy without glow, but when partially warm it falls through the cracks to some extent, where it's not cold enough to trigger the glow system, but not warm enough to start without glow. Easy start when cold but hard when warm suggests not an air ingress problem, rather an issue having to do with in-cylinder conditions (in which cranking speed does play a large role).
Marginal starter would be a good first step. If it cranks same speed cold and warm that does not guarantee starter OK, when the glow plugs are on it will start even while barely cranking at all.... On a couple I have had adjusting incorrect valve lash be the cure but a '91 should be hydraulic so not likely in your case unless someone put on a different head at some point. Cam and pump timing matter to some extent though presumably you have ensured that those are OK if it has ever had a belt while in your care.
Once all else is exhausted it comes down to compression, on a couple where they were just getting tired enough to have this issue but otherwise still ran OK and the owners preferred to continue to limp along with them for awhile rather than overhauling, I've had success temporarily adding a low-ohm resistor in the GP relay temp sender circuit, this has the effect of slightly shifting the controller's temp vs glow logic so that the temp threshold for turning the plugs on is lowered. Effective provisional cure, and leaves the option of driving it like that or doing repairs in the future, without burning up the starter in the meantime...