1990 Buick Riviera A/C

A/C stops blowing cold air time to time, specially on highway. When I accelerate starts again. Fan is working but no cold air. I checked the fuses/relays everything looks good. Checked the system pressure and the refrigerant is OK. I connected the fan directly but nothing changed.
ANY SUGGESTIONS!!!!!!

Thanks

Kind of sounds like a vaccum leak, but usually youll lose cool air when you accelerate not the other way around. Maybe your pressure switch is bad not letting the coolant circulate properly?

Thank you for the advice. I'll check the pressure switch.