Hi,
Your fan clutch could be failing and as a result, it isn't able to keep up with the heat load. Or, your temp sensor for the auxiliary fan may be defective, and so the aux fan isn't coming on when it should. Lastly, it could be the aux fan itself.
Start with the clutch, read Bentley's procedure on it to see if it's bad. Odds are, that's the problem if it has never been replaced.
This assumes that you have an otherwise okay cooling system, that is, fluid levels at normal, no air in the system, etc.
Regards,
Adnan