R134a usually just gives reduced cooling at idle -- it will blow cool air, not cold, and the fan will stay on high. This was my experience with the 300TE in Cleveland -- temp in the 80s, six people in the car, and stop and go driving with a bad visco clutch. Wasn't hot, just not cold.
Hot air would be something else, like the compressor cutting out.
My brother says ALL German cars, R134a or R12, have limited cooling capacity at idle -- his new Jetta TDI does the same thing. Has to do with a 6000 rpm rev limit on the engines, I think, so they run the compressor slower than US or Japanese cars.
Good thing, I'd hate to burn one up trying to pass someone.....
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!