The air conditioning systems on some 2002-05 General Motors SUVs (full list below) may be slow to cool in high temperature/humidity conditions. According to GM, the problem is usually intermittent, surfaces right after engine startup and is most prevalent during low engine and/or vehicle speed conditions. Insufficient airflow through the condenser due to a slipping electro-viscous fan clutch is the likely cause of the malady.
The fix is pretty straightforward: install an improved fan clutch assembly, part # 15293048. On 2002-04 models, however, you'll also need to have the PCM reprogrammed with updated software. The new calibration allows additional clutch engagement when idling or when airflow through the condenser is minimal.
Trucks that are susceptible to the problem are 2002-04 Olds Bravadas; 2002-05 Chevy TrailBlazers and GMC Envoys; and 2004-05 Buick Rainiers.