When diagnosing AC, always begin by identifying the type and model of the compressor. You mentioned a solenoid valve and it probably means it's an externally controlled variable displacement swash/wobble compressor. With these compressors, checking for pressures is pretty nonsense, because they can adjust the pressures in the system by themselves according to the cooling demand, all they have to do is reduce the swash plate/driver angle.
Anyways, it's better if you identify the compressor model (reach for the compressor in the engine bay and read the label, it's written here) and post it here, so that we know what kind of compressor we're talking about.
OTs are most likely out of the question here (yes, modern systems, especially ones featuring variable compressors, are pretty always based on a TXV, because orifice tubes are obsolete technology that can't allow for any superheat adjustment. How can you know it's TXV or orifice tube, without even looking for anything resembling one of these two devices or pulling any drawing at all? Look for where the accumulator is located. If it's on the high pressure side right before the evaporator, it's a TXV system and the accumulator is actually a liquid receiver, otherwise it's an OT system. Variable compressors don't really care if it's OT or TXV because they can adjust their flow any moment so no risk of compressor slugging even without a low pressure cut out switch, but TXV is always preferred because it's a more energy efficient solution).
The hissing sound from the evaporator's area, therefore, means one thing: refrigerant vapor is reaching the TXV inlet (only liquid refrigerant should make it there). This shouldn't happen. That means you're most likely on a low refrigerant charge and the receiver isn't storing enough liquid refrigerant.
Low refrigerant causes evaporator frost to happen fast (basically that little liquid that makes it through can't cover the entire coil extension, so that it boils only inside one part of the evaporator, near its inlet; the evaporator progressively develops frost across all its surface as the actual boiling location progresses along the evaporator's coils). Evaporator frost stops heat exchange and clogs airflow through the evaporator fins so that, more than no cold air, you should actually temporarily feel a reduced airflow. However, the AC system senses the evaporator frost (temperature sensor on the evaporator's fins in the case of the ECVDC) and reduces the compressor's cooling capacity (swash plate angle in that case, provided we're talking about an ECVDC) and this means less cooling and frost soon melting. And everything repeats like that. And that might be what you're experiencing.
Compressors don't like low refrigerant levels too: it means less oil flow around the system, so there's a risk of the compressor seizing due to a reduced oil return rate. It also implies higher superheat because that little liquid trickle inside the evaporator is absorbing more heat than it should. So the compressor is not receiving the same cooling effect from the refrigerant that reaches it. Add the lowered oil return, and the compressor is getting hotter than it should. And this can mean compressor damage.
Bottom line, go follow Jan's recommendations as soon as you can. Have the system inspected and serviced (which means having the refrigerant evacuated, system leak tested, vacuumed if no leaks found, then correct gas quantity inserted along with all the oil that the recovery machine pulled out of the lines). Having the system serviced by a pro is the only way of not doing any damage when dealing with variable displacement compressors. AC systems featuring one of these must be charged by refrigerant weight, not by pressure.
