It is not uncommon for a cooling system to have a diminished cooling capacity due to any of the following conditions (and sometimes more than one):

1. Thermostat not functioning in full range of travel. (I.E. might only be opening 60% of the way under high heat conditions.)
2. Scale buildup in Radiator. Using cheap antifreeze and not changing it at specified intervals will lead to Phosphate buildup (white scale) in the radiator which prevents thermal conduction and restricts water flow through the core. I advise using Volvo coolant, which contains no Phosphates and is biodegradeable after service life.
3. Scale buildup in engine cooling passegeways. Causes heat gradients and uneven cooling. In worst case conditions it can lead to steam buffering from heat pockets.
4. Failing Clutch Fan.
5. Failing Aux Fan.
6. Missing/damaged fan shroud or other ducting hardware (belly pan, air dam, radiator air scoop).
7. Failed cooling system pressure cap or other pressure relief due to a broken hose, etc.
8. Radiator, Intercooler, or AC condensor loaded with bugs/dirt and restricting airflow to the radiator core (I typically back flush my radiator core from the engine side, and clean the bugs out of the AC condensor with a dental pick every two years or so).
9. Improper mix of coolant (coolant mixed too lean or too rich for conditions).
10. Failing waterpump.

This list is by no means comprehensive, just a few ideas.
God bless and I wish you the best in tracking down your culprit.
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 258k miles.
'88 Black, 780, PRV-6, 148k miles.