It sounds like your " reserve capacity" is getting low.... a dozen things could be the cause...
I suggest that you clean and straighten the fins on your condensor and radiator...
This is something obvious which is different when going faster...
Also check out any aux fans you have...and and viscous clutches you may have... things like that could produce that symptom.....
Physics wise... the primary controlling factor on an AC system is ' air across the condensor' ....