84 euro 240d with parallel flow condenser and r12

[Rjinaz]

I have a 84 euro 240d which I acquired two years ago. The car came with an aftermarket ac setup. This past summer I overhauled it with a new sanden 508 compressor, expansion valve, reciever drier and a large parallel flow condenser and charged the system with r12. This combination has worked very well for me. I live in Phoenix. In the mornings ambient temps are in the low 90's, but I can get 40 degree air out the center vents on high fan speed and even in the mid thirties on medium speed. Coming home today the ambient temp was 112 degrees. Despite this I still got 50 degrees out the center vents on high fan speed and right at 40 degrees on medium fan speed. The one problem I have is that the ac system seems to work better at low driving speeds(20to40mph) as opposed to lets say 70mph where the air simply isn't as cold, not to mention the fact that if I drive at 70 for an extended period of time the blower speed slows down on its own an starts putting out warm air. If I return to slower driving , blower speed starts to pick back up and air starts to get colder again. Any Ideas as to what is going on here?