This one's got me stumped, '85 300D turbo. Last week WU and I went to visit my Dad up in Rockford, about a hundred mile ride. Took off up I-55 all nice and toasty ( it was about 15* out) after it was nice and warmed up I took it up to 75 mph and noticed a couple miles later cool air was blowing from the vents. Hit the temp wheel for some more heat and got non, looked at the temp gauge and it was up to 95*. We slowed down to make the tollway ramp (about 35 mph) and the heat came back on, temp also went back to 85. Got off at the next exit to take a look and found nothing odd under the hood, all fluids were fine, so off we went. Holiday traffic was heavy so we stayed under 70 all the way up to I-90 and had perfect functionality of acc. Then the same thing as soon as I hit 70 (about 3000 rpm) lost the heat, tried slowing to 55, no way didn't get heat again till we slowed to 35 for a toll. I have repeated this 4 or 5 times in the last week, always the same speed it goes out. Around town and under 70 the acc works flawlessly. Sorry for the long post but I wanted to give as much info as possible.
Any ideas would be greatly appreciated as driving in Chicagoland under 70 can be skeery!! and driving while shivering is no fun either,
Thanks. James

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