You can replace what you want, I'm just telling you that replacing the pressure switch or the expansion valve is not going to fix your problem.
A pressure switch failing would cause you to have no A/C at all, regardless of the setting that your fan is on. All the pressure switch does is let your ECU know the pressure of the refrigerant so that it can engage or disengage your compressor clutch. If the pressure switch is bad, your compressor would not engage AT ALL, and you wouldn't get any cool air at all.
Your expansion valve changes the refrigerant in the A/C lines from high pressure liquid to low pressure liquid before it gets to the evaporator, and just like the pressure switch, if it goes bad you get NO a/c at all. NOT intermittent A/C.
Also, what setting your fan is on has NO relation WHATSOEVER to the actual A/C system, as far as the components go. As long as you have your A/C on, your A/C system is cooling at a certain temperature regardless of what your fan speed is. In an ideal scenario, the only thing that changes when you change the fan setting is the speed at which the cooled air is blown out of your vents. Whether your fan is set at 1,2,3, or 4, the temperature in your evaporator core is the same.
My guess as to what's happening is something is shorting out in your control head, and causing either your fan to cut off or your hot/cool blend door to open (and thus allowing warm air into your cooling ducts).
I can promise you that if the A/C works on some fan settings and doesn't work on others, that it is NOT the expansion valve or the pressure switch.
But what do I know, I'm just an ASE certified driveability tech.
