95 E300D ac not cooling on highway.

[s_sprry]

my 95 e300d ac cools ok when sitting at idle (40-45 degrees out from vents) but the temp goes up 10-15+ degrees when on highway. I know the compressor is on when on highway because if I hit the econ button the temp goes up to outside temp or above. Also if the ac fan is on high the temp goes up 10+ degrees. Since it's cooling ok at idle then freon level should be ok? Could this be a bad expansion valve or bad mono valve or weak compressor?

Thanks

 

[television]

to test the mono valve turn the air con off and set the controls to the lowest setting and the air coming in should be cold when the engine is at 90c, if the air is warm then the valve is not closing