erratic ac compressor

[unleash the beast]

I've searched and read a ton of ac problem threads and none seem to be like mine. I have a 97 240sx and the AC has been kinda poor this summer cool air comes and goes, so I figured I was losing refrigerant got a can to add to it but wanted to check the pressure before I put the new can in. I hooked up to the low pressure line and it read about 45 psi with the compressor clutch engaged, the the clutch disengaged and the pressure slowly crept up to about 180 psi, blowing hot air the whole time, the the clutch would engage and slowly start to blow a little cool, then after about10 seconds the clutch disengages and hot air blows while the pressure slowly builds back up for about 45 seconds to 180 psi and the cycle continues. So I got the system empty and filled with proper amount of r134a, at idle the compressor was running and the pressure at 45psi, and finally some cold air, but as soon as I drive it the compressor kicks on and off like crazy, sometimes every 10 second, and sometimes like a strobe light, on off on off on off and the air is barely cool. I had originally thought a problem with the expansion valve was possible until the extremely rapid on and off of the compressor, could this be something else?? Any help would be greatly appreciated, the past 3 days here have been right at 100 degrees and about 110 percent humidity, I'm dying!!!

[s13sr20chris]

i would lean towards the expansion valve.