If that were an old R-12 system, I could probably figure out what's going on with a set of gauges and a duct thermometer. However, I have never even touched a R-134 system. I assume the principles are the same, but the "standard" pressures may differ.
With R-12, this was expected behavior - with the system on, the low side should run at ~ 30, the high side at ~ 160. The electric engine fan should be running constantly with the A/C on. Running the engine at ~ 2000 rpm, the low side will slowly drop, and the compressor should cut off before the low side drops to 20. The pressures should go toward equalization, then the compressor should come back on. If the high side goes over 200, either you are overcharged or there's a blockage in the refrigerant system or there is insufficient airflow through the condenser. If it gets really too high, the fusible plug in the accumulator will blow out. Sticking a thermometer in a dash vent should give you ~ 40 degrees, but it will rise if you drop back to idle. The hotter it is outside, the higher the high side pressure will be, and if it's much over 80 degrees outside, the dash temp may not get down to 40.
If the low side pressure drops below 30 and
stays there, the evaporator will freeze up. This is usually a faulty low pressure cutout switch. The lower the low side pressure, the colder the air will be.
A trained tech can observe gauge behavior and tell if it's overcharged or undercharged.
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The system is fully charged with gas and the compressor cycles on and off exactly as it should.
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Exactly how have you determined this? You may want to bite the bullet and take it in and get it properly serviced - which is leak test the system, evacuate it, and recharge it exactly by weight.
If you think the evaporator fins are clogged with crap, you *should* be able to get at it without cracking any fittings - it's either under the dash or in a doghouse on the firewall, probably not too far from the heater core. A repair manual should tell you where and how.