There's really only a few things that could be happening:
-You don't have enough fan flow due to it not kicking on, dying motor bad clutch etc. This would manifest at idle.
-The radiator is blocked up internally, the cooling system is too small or a tstat is blocked. This could manifest at idle and would certainly at speed.
-If the timing or fuel mix isn't right you will generate extra heat that complicates the diagnosis.
-You don't have enough fan flow due to it not kicking on, dying motor bad clutch etc. This would manifest at idle.
-The radiator is blocked up internally, the cooling system is too small or a tstat is blocked. This could manifest at idle and would certainly at speed.
-If the timing or fuel mix isn't right you will generate extra heat that complicates the diagnosis.