the oil pressure will always drop when the car is at an idle, when you accelerate, the oil is getting pumped through the engine at a higher pressure, so the gauge will read a bit higher than at an idle. as far as the AC and power loss, when was the last time the system was recharged? it could be low on refrigerant and shutting itself off under load, but still have enough to want to kick back on. run the AC on max with the fan on full at an idle and keep an eye on the clutch for the compressor (part in the middle of the pulley) and see if it kicks on and spins, or if it just spins for a second then stops and repeats that process. if it stops and starts repeatedly, then you should check to see if the system needs recharged, if not, then it has enough to handle an idle, but could be low enough, that under load, it thinks its low, in that case, i would have the pressure checked just in case.
hope that helps somewhat.