90 300TE -blows hot air with AC on

Hi, my 1990 300TE after some amount of normal AC operation, will suddenly start to blow hot air at the same time as the cold air. This happens at
random times regardless of the setting of the temperature wheel or push buttons. I've taken the control box out and cleaned and resoldered any suspicious connections- didn't help. Any suggestions or pointers would be very much appreciated- thanks!
Tom Sharples
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