I have the same car, and on my vehicle there is a separate relay on top of the valve cover that cuts off the A/C compressor once the throttle is depressed sufficiently. It has nothing to do with the transmission "kicking down," although people frequently think that's what it does. Now it may be the relay on the valve cover sends a signal to the actual KLIMA relay to shut off the A/C, but there are 2 relays. I have had issues with the relay on top of the valve cover before causing intermittent problems with the compressor operating properly.

Sadly, there are not that many mechanics around that know the A/C systems well in these cars, and having the CA version with the KLIMA relay adds yet another layer of complexity. Many of them just want to add freon and send you on your way. The system will never work amazingly well with R134A, but it should certainly work better than you describe. I daily drive mine in Austin, TX, where the summers get hot and humid, and mine keeps me more or less comfortable. Perhaps the "good repair shops" forum will have info on a place in your area to take it to be diagnosed?

Good luck,
Chris

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013