Along with all those great suggestions - did you also check to see if the evaporator - being in the passenger compartment - it's condensation from all that air conditioning - is draining out? There is a drain port in the plastic housing that pushes down into a low spot under the front dash - acts like a spout - but if it's not properly seated - it can leave itself pinched or plugged to slowly drain out water but still leave enough inside the blower housing to fool a sensor into thinking it's too cold so a temperature module tells it to relax a little and let it warm and dry out any icing condition it can cause.
A Properly mounted and working AC when it operates will easily drain out...also would leave a puddle of water behind the engine - about mid-point in the vehicle centerline - on the ground.
- If it doesn't - it can affect the temperature sensor "sender" that the Climate controller uses - for it may see "chilled" water splashing onto it in the tight confines of the blower unit. You may even hear water which may make the dash creak and shift - or you'd feel some moisture and wetness from the floorboard and mats absorbing the water until the drier air can dry out the carpets due to the lowered humidity the air is.
Another condition is dealing with the quality of the fuel you're using - lower grades of octane are offset by higher alcohol content that reduces the power the motor can offer to work against the load that's placed on it. So that means, it has to open the throttle and offer more timing advance to offset the load to keep idle and emissions in check. IF the throttle angle goes up due to that load - the A/C is oftentimes told to turn off by the PCM so to save the gas and fuel economy (ECO-modes some vehicle dashboard setting provide) as a means for Fuel economy.
If you think plugging... Check the rear facing vent under the passenger seat for this - it may get wet from the vent deflector/director blowing out water along with that chilly air. The deflector is the last segment of a long line of duct-work - if it's not wet, it can leave enough of a gap so water can flow into the carpet padding.
The biggest clues would be from when you turn off the air - and turn on the heater side and tons of hot humid, smelly air rises out and fogs the windows requiring you to quickly turn on the defroster to remove the fog. You'd also feel or hear the blower kick (speed up) higher in coasting or lighter load modes because it's using the motor as an engine brake so it tells AC to turn on to help slow down the car as part of that braking.
Else try to use a higher octane to help with idle speeds as it may offer more power to offset the PCM's desire to work on the Fuel Economy side than try to keep the idle speed higher because the pump pulls power from the motor.
This also goes towards the belt, electrical connections that may corrode due to age - and if the battery is weak - it may not have enough power to supplant the electrical demands of the EPAS (poor alignment - extra wide tires - brake drag?) so the PCM decides to shed loading - the first things to go are AC - because the Alternator is telling the PCM thru it's LIN system - "You're asking for this - I'm at that now...what else can I do?" - and it can't find a solution except to drop features and lessen the loading on the motor giving you sluggish performance. So to offset that, you might need to review, repair and or replace the battery and it's connections to the system to improve the electrical side of the performance - especially if this is happening and you didn't notice this before earlier when it was newer and you had the vehicle with fewer miles on it.
