Do you have the dash control set on recirculate or fresh air? Perhaps the fresh air is hot and humid enough that it's tougher to cool. Also, at faster speeds, you are circulating engine anti-freeze at a faster rate/higher pressure. If your heater control valve is allowing hot coolant into the heater core, you'd be canceling out the A/C. Try pinching off a heater hose with a set of vice grips and see if cooling improves. If it does, either adjust, replace heater control valve OR install a simple ball valve shut off in the hose to the heater core. Shut it off for summer and when you don't need warm air for defrost.
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Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI
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