Ok found these 2 threads which you might read through...........
http://forums.nicoclub.com/ac-works...e-day-time-when-its-hot-it-stops-t350567.html
http://community.cartalk.com/t/ac-clutch-stops-engaging-in-hot-weather/79723
This next to last post confirms that someone fixed a similar problem this way,
"Repair update: Ok, so I replaced the thermostat (I think it was the original one with 190K miles), did a
radiator flush (didn't know you have to bleed air from the engine after a radiator flush btw), replaced the cap. It worked better but still did not fix the problem:compressor still turned off during the hottest part of the day. Took it to a friends house and he put the gauges on during this super hot part of the day. He said that the pressures were way too high and suspected that the system was overcharged. Took some gas out (luckily, the 1993.5 version of this car is the first year that they started using R134), still didn't fix the issue.
So, check this out...he sprays the front of the condensor\radiator with a hose (not too much pressure as to not bend the fins) and walaa! Pressures go down on the gauges. So...I did a more thorough job and between the condensor and radiator there were cloges of dirt! I sprayed everything down (gently) and the system works great now. I have to have it on full blast during the day but the AC compressor is cycling on and off now and cold air is always blowing. What a simple fix! I'm grateful"
If you do try the last thing be careful not to spray the fins on the front of your consensor to hard as you might bend them. There is a fluid you can buy and spray in the condenser fins to clean them but a stream of water will clean it as well.