oil pressure gauge

[bluefin59]

Hi guys ,i posted a while ago about a dropping oil pressure problem and it has been ok since i changed oil and filter but the gauge on the dash of my ghia has been a little irratic sometimes falling a little with the air on on a hot day . Sometimes after i have driven with the air on when its a stinking hot day over 30 at least the gauge will drop away but when i turn the air off it will rise again to almost normal levels . Yesterday i drove without air and the gauge went up over its normal level so i turned the air on and it fell back to normal again ,do you think its a sender problem or maybe a regulator or what ever control the power around the car . I only put a new battery in last year or could it be the aircon but it has always worked fine ,if anyone has come across a problem similar could you please help as i am a bit confused where to go with this it is so intermitent that i am not sure what to do thanks in advance ...matt