Does the aux fan come on when you go city driving? If not, check refrigerant pressures because if the pressures and down, it will not kick the fan on, then it won't have adequate air going through the condenser. Therfore, decreased cooling.
Bottom line, check pressures. Pressure values differ due to ambient temps- might need to call the dealership for the values for your particular vehicle-- One of the things they don't charge for-information.
Best of luck
1978 280 CE silver
1986 560 SL black
1987 300 SDL cream
1994 SL 500 brilliant green
1997 E 300d dessert silver
2005 FORD F250 Superduty Crew cab 4x4