I replaced the duovalve with a used unit with no change. It still misbehaves in hot conditions.
Curiously, when I set the fan on low speed before I engage the A/C and then slowly increase the fan speed, things seem to act normally. It is likely to misbehave when the fan is on high when I engage the A/C.
I'm going back to the shop that replaced the compressor today to have them check the charge.

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1999 E320 (gave to daughter) 250K
1994 F150 (may be the end of its road) 240K
1989 560 SL (Reassembly after paint job) 160K
1986 560 SL (deceased., gave it's life protecting my wife)
1988 300 TE (departed)
1994 945T (still running strong with an upgraded cam) 349K
1986 242Ti (deceased)
1968 GT500KR (under restoration)