I have just done my a/c as I was also having problems - on hot days and when the car was stationary the a/c would not work effectively. I live in the tropics so this is important.
Investigation revealed that my evaporator (the small radiator type thing inside the heater box in the car) was leaking - this was discovered with some dye in the a/c system and an UV light. This meant the the system could not hold pressure.
On the right hand drive cars you can not get the evaporator out without removing dash and heaterbox
So I ripped the interior of the car apart, installed a new evaporator, got the system recharged and checked for leaks - no problems.
My aux fan in front of the condensor was working just fine and the condensor was clean and free from rubbish - very important to have air flow through the condensor and that the fins are clean for max cooling.
I now have
ICEY COLD air con
So I guess what I found through my experience with a/c is the important thing is:
system pressure - check for leaks (the sight glass in the receiver/dryer will show bubbles) if necessary use dye and UV
cooling of the condensor - good aux fan and ensure the condensor is clean and that air flows through ok.
Whilst I was playing with the car I also checked the radiator fan and found that the clutch was not working very well - could stop it with a rolled up newspaper quite easily. So I installed a 16" thermo fan
It runs from the 91degree switch on the radiator and is powered even when the ignition is switched off so it will continue to cool even when the car is parked. It has "S" shaped blades so it is very quiet and pumps heaps of air
