McQuay 80 ton air cooled cond unit (mod# ALP080C ser#55L8132201 2 35HP copelands 1 per stage) serves AHU feeding VAVs. For as long as ive been serving the particualr building the 2nd stage of above said unit. Has a history of intermiitent High pressure tripping and it has to be reset. Now im going to get the obvious out of the way first off, coils are clean enough to eat off, all fans work and all controls to stage fans on ( first runs all the time on a call, 2nd on pressure switch, last 2 on OAT) as they should. Even in these low OAT (northern MN) the mech clg is enabled at about 57* and intermmitngly even in this weather has to be reset- even though the high side pressure isnt even 200 psi- there is NEVER a high pressure condition while checking. The pressure control is a NO switch that closes on rise that makes the holding relay coil until reset. Ive replaced both the relay (R2 on diagram) and the pressure switch (HP2) to no avail. The only other thing would be intermmittent shorting of wires 203 and 204 due to vibration making the holding circuit somewhere and ive inspected every inch of wiring on said circuit and found nothing. My next course of action is to install an outside manual reset HP switch so it is protected and put it in series with the LPC or wherever to see if it still happens over time to see if there is in fact an intermittent HP problem or if its with the circuitry. Thoughts or similar experiances?