Why does my AC not work when I'm driving? 98 isuzu rodeo

I have a 98 Isuzu Rodeo I have been trying to fix a problem with the AC. It seems to run fine when idling, but when driving it starts to run poorly. I have manifold gauges, hooked them up to the low & high pressure ports earlier this morning. Static pressure was 97 on low port, and 100 on high port. Outside temperature was 75, but I don’t know what the humidity was. I had the windows closed, AC on coldest, fan turned on full blast, recirculating. I stuck a probe thermometer into the center vent, and after about 10 minutes the air was blowing at 40 degrees F. The compressor was spinning, and low pressure was 40 and high pressure was 225. I ran it like this for about 15 minutes, no problems. Then I wedged the throttle with a pencil and that pushed the RPMs up to a hair over 2000. Then the compressor started shutting off, then on, but not in any kind of rhythm that I could detect. The low pressure would drop to 23, the high pressure would jump up to 400; then high pressure would plummet to around 220. Engine coolant level is normal, both fans are blowing, K & N air filter just got cleaned, relay seems good. The check engine light was on with a reading about the EGR valve, which I have cleaned. The engine light has not come back on. I know this is a lot of detail, but I’m trying to be thorough. I’m handy and mechanically inclined, but out of my depth at this level of AC troubleshooting. Trying to save several hundreds of dollars by doing it myself…

400 psi on the high side is the cause of the compressor disengaging. Am I correct in assuming that it’s when the pressure gets this high that the compressor shuts off? That would be the high side switch (or some such device) cutting off the compressor before something blows up. A rule of thumb is that on a hot day with the compressor running you would see high pressure in psi approx 3 times ambient temperature in degrees F. So if it was 75*, I’d expect high side operating pressure to be about 225 psi.

Poor air flow across the condenser, restriction internally in the condenser, discharge line, possibly the expansion device failing, are the first things that come to mind.

With the high side going high, then leveling off, check condensor coil temp up front. If the temp spikes when up when around 400 psi, the problem is airflow. Check your fan clutch and make sure your fan shroud is in good shape. If the temp slowly climbs as the pressure spikes to 400 psi, you have a slow or dirty expansion valve.

I think @asemaster and @BustedKnuckles are on the right track. I had a similar problem with a Ford Aerostar van. The AC would work for a while and then quit. The technician at my independent shop noted that when the AC would quit, the high side pressure would spike. He wheeled his big shop fan in front of the grille and turned the fan on high. The high side pressure immediately dropped and the AC started working again. In my case, it was a slipping fan clutch that wasn’t allowing enough air flow through the condenser even though the blades were turning.